Clearly, we need to measure in TIE Fighters and Z95s.
MoV is awarded in half healths for anything 13-24pts. 25-36pt ships give out MoV in thirds; 37-48pt ships give out MoV in quarters. This way everything is perfectly fair because everything loses MoV at the same rate as a swarm. Perfect, right?
(no)
... or we could just declare it to be any ship worth more than, say, 30pts. Or 25. Pick a number and run with it.
It's not the size of the base or the HP of the hull (Remember killing 1HP Whispers, kids?), but that you can load so many points onto a single hull, and that on said hulls you've often invested a lot into making it hard to kill outright.