An ork has more gorilla-like limbs and posture compared to a human (even a marine), so:
a) an ork in marine tactical armour looks weird, like a ork painboy experiment gone wrong, which is usually a bad thing unless that's what you're trying to represent
b) an ork in termy armour looks more believable due to the bulkier limbs, even though the pose is still too upright, but termy armour is easier to bulk up (as per tgjensen's example above) to look more like it's been captured and 'improved' by mekboys and their grot assistants to create something counting as mega armour.