The problem is that people are confusing mass with volume. If these mechs were all solid throughout, of the same material, like statues, then using a mass to volume ratio would apply.
But they are not solid, the internal spaces of a mech are just that, empty spaces. Some mechs were quite compact, with little space between components, others, like the Orion, were known to have large internal spaces, making maintenance exceptional easy, and loved by the techs for being so. Some mechs had cramped cockpits, others had basically suites with bed, cooking and washing facilities.
Also, all the components in a mech are not of the same density. Myomer would be less dense than armor plating. The larger the surface area, the more weight in armor that would be required, since armor "tonnage" is related also to thickness/volume/mass/density.
A larger frame would require more tonnage in dedicated to the skeletal structure of the mech as well. Larger mechs would not just require "longer" and "wider" skeletal frames, but also thicker skeletal components, so weight of skeletal frames could not be scaled linearly.
Whereas other components, weapon systems for example, would be a composite of different materials with different densities, and still packaged with internal spaces within them. A gauss Rifle's volumn is more than the weight of it's components, the bore of the gun is empty space, as well as the different densities of the components, say, the capacitor volume/density compared to the gun barrel.
With engine sizes, a mech has to have enough space to fit it's max rated engine, regardless of the tonnage/size actually utilized, and with sufficient space to install and maintain.
And as stated prior, there are spaces between all these components, to allow access for maintenance.
So, only utilizing mass as a criteria for volume is insufficient to accurately scale battlemechs, it's the "easy" solution, but has now generated the absolutely ridiculous scales we currently have received from PGI
Edited by Lupus Aurelius, 25 June 2016 - 07:54 AM.