My Black Templars Marshal is a bit of shortarse. Its not his fault, he's a very old sculpt and so he isn't on quite the same scale as modern Space Marine miniatures. He stands a few millimetres shorter than the Sternguard I'm painting alongside him. He also has a strangely boxy head.
There are no aesthetic issues, there are only background opportunities.
So this Marshal, who I haven't named yet, stands shorter than other Astartes because he's centuries old and has charged headlong at so many massively powerful enemies of the Imperium he is now made mostly of cybernetics. His legs in particular have now been reduced almost entirely to artificial replacements. His head is boxy because his skull now is a metal box. The model even has a rebreather over its mouth which I've decided is his mouth: an artificial breathing rig and voice augmitter installed after an Ork Warboss ripped his lower jaw off.
It isn't an easy life in the Adeptus Astartes.