Minmax's armor is likely designed to fit any general Human, maybe even Elves or Dwarves with some adjustment of straps and bindings. We don't even know for sure if that armor was designed to fit female humans, it may specifically be designed for human males, we don't know.

Even if that's not the case, i'm sure the armor wouldn't conveniently fit if Minmax had say, four arms, or a pair of wings sprouting from his back. He'd at the very least need to modify it in order to accommodate his needs, which is something Big-ears just can't do with a solid metal helmet.

I severely doubt that his armor would conveniently fit the tall blue Minmax or the Nianderthal Minmax.