The larger enemies deal significantly more damage than the smaller enemies (6x higher than smallest enemy) to compensate for their lack of speed, and as you mentioned you do still need to shoot them for fuel. You could preserve fuel by choosing to limit your movement while only large enemies are on the screen, and pick off one at a time to prevent getting swarmed by the little guys. As an aside, they still split into smaller enemies if they run out of fuel. I realise this is counter intuitive, and had considered making this not the case, but just didnt get around to it by the time the deadline was close!
Thank you for your feedback! Love the idea of non-enemy units. Perhaps the enemies could also target them and try to grab the fuel so its a bit of a race. Could be fun.

