--- Allow me to explain, and please bear with me as I'm quite bad at doing so. Dyscalculia is a bitch and half and then some. 
I'm going to ignore the Range as a function of distance because while relevant in practice it is irrelevant for the purposes of expressing this concept. Range in terms of distance is still governed by how long it takes to get there and is thus arbitrary with respect to Deployment Times and Burn Times, AKA "X Days at Full Power" The following examples are but two of the possibilities that I know of.
Example A:
Let's say that Ship A has enough fuel for 6 Months worth of Burn Time and enough Deployment for 6 Months worth of operations.
Now let's say Ship B has enough fuel for 3 Months worth of Burn Time and enough Deployment for 6 Months worth of operations.
Finally, let's say Ship C is a Commercial Tanker for Ship B, due to it being Commercial it only needs 3 Months worth of Deployment Time and carries 3 Months worth of fuel for Ship B.
Example B:
Let's say Ship A and Ship B have the same amount of fuel and Deployment as before, but now we have no Ship C in the equation.
Ship B is designed to spend 1 Month at reduced power, enabling it to travel for 6 Months at that speed. It then can use it's remaining fuel for high-speed maneuvers.
So while Ship A can go the full 6 Months at full speed, Ship B cannot and does not, instead dividing it's consumption between a strategic "Cruise Speed" and a tactical "Combat Speed"
--- So in Example A, Ship A and Ship B both carry an amount of Fuel equal to their Deployment, but Ship B offloads some of the fuel to Ship C. In Example B on the other hand, Ship A and Ship B are still carrying an amount of Fuel equal to their Deployment, but Ship B allocates it in such a way that it carries less Fuel to achieve the same endurance at the cost of having less speed available to it overall. In either Example, an overage of either Fuel OR Deployment would result in wasted tonnage, with the exception of redundancy with regards to Battle Damage or time spent loitering on station. Both of these, however, are deliberate design choices rather than some natural or normal thing. Exceptions, rather than the rule, so to speak.
I fully understand what you are tying to say here but let me explain why I think this is faulty logic, for the most part.
If you have a ship that carry fuel equal to deployment I think that is fine (in some cases)... if that is the role of the particular ship. It could be something like a fighter or FAC for example. Even here you sometimes want more deployment than you have range for fuel so you can extend their range further when needed. Se below why this can be smart...
First of all, fuel can take up an awful allot of space so if you have a ship with an intended deployment of say 12 months you might not have room for much else with that much fuel as well. If you offload fuel to a tanker you suddenly have a ship that require no maintenance and can run fuel runs non stop for as long as you keep the ship around. The same ship can probably keep more than one ship with fuel as warships need overhauls/upgrades at certain intervals. Your commercial ships don't require any maintenance facilities and are much cheaper to maintain which means you can keep a larger fleet for less cost.
If you also look at the practical side there are a few points to make... any fleet stationed inside your territory can utilise refuelling points to get on stations thus will not need tankers for travelling inside your space if you space out refuelling points close enough to each other.
You can station a fleet quite close to any hot spots while you are scouting for potential targets, thus they need much less fuel than they need deployment time.
It is in my experience common to have a fleet camp at a JP for a pretty long while, perhaps a few system away from any refuelling stations while you send scouts ahead a few systems over. You don't know if there will be a fight or not or when... but you want the fleet close by.
Scouting in general can take up a considerable amount of time, time allot of ships either are just sitting around doing nothing or sneaking around trying to find the enemy.
Quite often you don't need more than a fraction of the total range of a ships deployment and that is true no matter how you build your ship... in your case you waste both deployment and fuel for the range you need... others just waste some deployment space. If is far cheaper to waste deployment when you don't need it than it is fuel AND deployment space if you don't need it.
We can make some more practical examples...
I have a Destroyer (in an old campaign) with a deployment time of 9 months who will burn its entire fuel storage in 43 days for a total range of 16mkm. It's nine months of deployment takes up roughly 5.7% of the ship space and their fuel tanks about 6.5%.
If I made this ship have the same range as deployment (using the same space and change nothing else) it would become deployment time of 2 months, and a range of 21mkm.
This ship would no longer be able to fill its function properly...
If I instead changed it to keep 9 months of fuel I would have to increase the fuel tanks from 6.5% of the ship to roughly 40% of the ships space into fuel. Given that this particular ship already have about 35% dedicated to weapons and 15% to defences you would have to strip off most of the ships mission tonnage just to make the ship able to run constantly for nine month which obviously you are almost never going to need, if ever. At this point offloading nearly 34% of a ships weight to a commercial tanker seems like a smart move. Obviously you would need a more efficient and bigger engine at this point... so it would not be as bad as stated above.
The point here being that range is most often completely detached from deployment. The range a ship need from a design perspective is always going to be the distance it need to travel from any source of fuel and it's target, including battle manoeuvres.
The deployment a ship need is also depending on its role and how long you might expect the ship to be able to perform in the field away from a base where the crew might rest for a considerable amount of time.
You could argue that you might build ships with more efficient engines... but then you still need more space for engines while you still don't need more range for battle manoeuvring. So you still end up wasting space on the ship. Smaller engines might require twice the amount of fuel to run, but fuel consumption for warships should rarely be a concern from a strategic perspective. Having more guns pointing at the opponent should be a higher concern overall. There obviously is a break point and for me that is the minimal amount of battle manoeuvre range my ships need. Depending on role that can usually be from 10-30 billion km in range for a normal fleet vessel.
The most optimum rate if engine to fuel are generally 3/4 engine and 1/4 fuel tanks... just decide what range you want the ship to have and build the engine to fit into that ratio. You might not always be able to fit the optimum rate for different reasons, especially if you have ships with different needs and having to use the same engine.
