[Andrés]

Yes, but the point of QNP is having engines per move proportional to ship sizes.
For example you have many ships with engines per move going from 1 to 255 engines per move. You're right actual tonnage of the ship is irrelevant.

That doesn't sound too bad, but the largest ships using whatever combination of engines to get 255 of thrust will move at 1, any attempt to make it faster will generate an error.

If you want to make the fastest ship have a maximun of 4 movements, you'll need engines per move of 255/4 ~= 64

Then your largest ship can only be 64 times larger than the smallest one, you can't even make the contrast between standard escort and dreadnought, much less add extra sizes.