That's a bug with a bunch of mods, possibly due to a conflict. I have the mass set appropriately in the mod but it gets jacked up anyways. It's honestly fine though, they're meant to have a bunch of mass anyway, in order to be able to compensate for the lack of speed or ability to run. The steam suits are meant to support weaker infantry, and be able to wade through and out of a blob if they need to, not be sent into hell as shock troops or something.

Actually you know what, now that I think about it, it might be their special ability doing that. It adds mass as a flat bonus that might be multiplicative instead of additive. That'd make sense, I'll have to take another look.