Is he a little shorter than the Arms Micron figure in robot mode???

If anything he looked a little taller.
There's some quite serious cheating going on with this new figure, but that's kind of par for the course with PRIME and when the end result looks that good I won't complain too much.
Seeing this review has added weight to the idea that I may not get any of the Beast Hunter versions of returning characters (except Smokescreen and maybe Optimus and Magnus). Most of them have significant differences between their standard PRIME and Beast Hunters appearances, but that rubber wreath of Smokescreen's makes almost no difference. Then again I suppose Rhinox, Waspinator and Inferno went the whole of Beast Wars in the same body while everyone else upgraded once or twice. May just get the ones I like i.e. not Beast Hunters Ratchet.