The Fluid Wagon looks properly scaled, but the Loco and Cargo Wagon doesn't.

It’s intentional that the locomotive and cargo wagon are stretched to correct proportions. The fluid wagon wasn’t possibly to stretch without the tanks looking oddly oval, so it is enlarged proportionally. I think that makes it look reasonably okay, if a little too wide to really fit the rails.

Don't worry about it being too wide, typical American boxcars are about twice as wide as the bogies they sit on. If you could make an option to make the Cargo wagon and Loco as proportional as the fluid wagon, that would be nice but no pressure.(also the standard factorio trains are puny).

Locomotives and cargo wagons are 3m wide, more than twice standard track gauge. AAR maximum loading gauge is 3.25m (10’ 8”) wide. The width is realistic.

The Chonky Trains fluid wagon is over 4m wide, too wide for any standard or even broad gauge railroad in the world.

My target for Chonky Trains is the EMD GP40 series, at 18m length and 3m width. Unfortunately the curve radius for Factorio track is fixed and 18m just looks comical, so I settled on 12m length.

understandable