Engine weights and power output are not linked, using base game rules and comparing to physics. AKA, a 40 ton mech moving 8/12 uses a 320 engine. An 80 ton mech moving 4/6 uses the same engine. However, engine power needed for speed/acceleration isnt linear to mass IRL. If I take a car and remove half of the non-drivetrain weight, so its an engine sled, it wouldnt have double the acceleration and top speed.
If the engine weights were linear, but your engine rating was tonnage*speed^2, we would get a much better curve. As it is, the 10 ton mechs and 5 ton vehicles are just FAR too fast, as the engine scaling, like Nerdi points out, are not properly scaled.
As for the engines themselves, a 10 rated engine in game, in like a trailer, can power infinite medium lasers, until the weight of the trailer and cooling system, not the output of the engine, is the limiting factor. So even the smallest engine has 'infinite' output, its the other stuff like transmission, drive train/myomere, ect, that should be a majority of engine weight. Bigger engines have more efficient cooling, but only by a little bit.
Like, if the engine formula was .5 * tonnage * walk^2, a cicada would have a 1280 engine and a zeus would have a 640. The Cicada engine would be 2x as strong, which makes more sense as it goes twice as fast, for a 4x energy cost, at half the weight.
It also would mean the joke speeds on things like savanah masters would get tonned WAY the heck down, into a much more gameplay friendly space. instead of 15/23 being the lightest possible engine, it would be close to as heavy as a 45 ton mech's engine moving 5/8--forcing those too fast units to have more reasonable speeds.
Edit: to be fair, if we switched to a more energy correct engine/drivetrain weight, we would need exponential structure and armor costs. Like how a cicada's engine should require more then a Zeus, the Zeus's structure and armor, point for point, should cost more then the cicadas. 4 tons of structure supporting 40 tons means an 80 ton unit would need 16 tons of structure, since support scales in cross section area while weight scales in volume. At a certain point big things bones are not wide enough to support the weight due to this scaling issue.