Smaller critters are harder to hit... Larger ones easier. That bit is already built in. Smaller critters (again ALL things equal) do slightly less damage. Larger more.
Then variable HP would be consistent.
Right now, the base Health mechanic is figured by Stamina + 10 + Specialization Mod. Specialization mod ranges from +14 for caster types to +18 for melee types. The stamina (for PCs) ranges from 0-10 with 5 being "base". Some critters do exceed those boundaries.
It looks like starting character are going to be pretty hard to kill. I remember one Gamma World edition that required an average of 35 arrow hits to kill a first level character....
Currently, a Very Small (no bigger than 1 ft.) creature has a Health mod of .5
Small critters (3-4ft) have .75
Medium have 1
Large (8-10ft) have 1.5
Very Large (12-15ft) have 2
and Huge (20+) have 2.5
Not as big of a difference as I had feared (cat=2, elephant=50). This may actually be understating the difference a bit but to make that determination, I'd have to playtest the entire combat system.
So a goblin, being small, averages out to around 30ish Health. A human fighter is around 40 Health. The goblin is harder to hit, needing a 15+ on a d20 (not counting any bonuses the attacker has) to hit. A human on the other hand is around 9+ to hit.
I'm somewhat concerned that you will hit the human twice as often as the goblin but the human doesn't have twice the HP. That's a net advantage to the goblin.
Armor does not stop you from being hit (stupid D&D), but once you are hit, reduces the damage you take. Some creatures have an innate armor value (AV). Some have a mitigation against a certain type of damage (skeletons vs non-crushing, demons vs fire, etc).
That's good.
Does wearing armor make you easier to hit?
Can a well placed hit bypass armor?
My gut says that a smaller creature should be easier to kill IF you hit, and a larger harder. It's just figuring out the correct ratio.
Maybe create a "hard to kill" table for working it out. Set humans to 1 and group creatures based on how hard they are to kill vs. a human. Then figure the To Hit vs HP to make creatures fit that mold. So, if a cat (for lack of a better example) is deemed as hard to kill as a human then reduce its HP by the same ratio as you increase its To Hit.
You might want to add in the quality of the hit. Thus, you are more likely to connect a grazing blow to a cat than to a human. This is more complication, however.