: I like to think that the progression was driven by the need for power and
: protection. Mk. I required tethered power and was underarmored, so they
: bulked it up for Mk II. The second Mark still couldn't mount a large
: enough power source and the fuel to keep it running in the field, so the
: Mk. III was made larger to accommodate a wireless energy receiver. And
: since walking in the Mk. II was like walking in a fat suit with a saddle
: between your legs, they just made the Mk. III large enough to house the
: pilot in the torso.
: At which point an oversight committee saw the diminishing returns and mission
: creep coming and nixed the whole project.

If it hasn't been addressed already, I think that's a good explanation for the even more mech-like stuff-- they decided to fork the project, with the goals oriented around power armor going to Mjolnir and the mech-oriented goals going to the Mantis.