In any way, I would really like to avoid bigger ships having a huge advantage in being both fast and with good range... as much as possible. I would hate to see 10.000 ton fighters with 6b km range.
Isn't that the case currently too though?
A larger ship with 50HS engine will have twice the range at same speed as a fighter.
But there are lots of reasons not to pursuit it, for example you can't build in FTR factories, you can't equip with FTR beam FCs, it will be spotted from far far away due to it's size, can't be repaired in carrier hangars, the engine research cost is prohibitive and the list goes on.
Overall I'm not very worried that fighters would suddenly suck because they are made to consume 1.5 - 3.0 times more fuel since many current FTR designs get away with 10 ton ( 0.2HS ) fuel tanks ( that would need to be replaced with 20-30 ton fuel tanks or maximum 4% of 500 ton weight ).
Range of fighters is often enough as long as it's high enough to get off 2-3 strikes before any enemy can close distance and you don't really need many billions of km or days of range for that.
If it should be a problem I would prefer mechanics that make bigger ships easier to hit to balance it, but I guess restricting large engines to a bit lower power mods could make sense seeing how it's already the case with smaller missile engines getting 2x the maximum amount.