
This should help a bit! When my phone recharges, I'll send a more detailed comparison pic.
From left to right: original Chronicle orc/black orc; Grenadier orc (now available from Forlorn Hope and Mirliton); Mothercrushers' champion (bigger than the troopers); Grenadier giant orc.
Basically, the Mothercrushers work perfectly with the Citadel black orcs (the bigger, slottabased ones) and Eeza Uzegod's Death Commandos. They also work fine with the normal Grenadier orcs, although those tend to be a bit smaller (and more crouched). The Citadel types - including the Mothercrushers - generally look like the biggest and toughest of the bunch, but they work well together.
The original. solid-based Chronicle orcs (later renamed black orcs) are very small; barring the odd champion, they'd look lost on 25mm squares. I base them on pennies. The Chronicle hobgoblins would mix in fine with the Mothercrushers if you like a bit of facial and tonsorial variety.
The Grenadier giant orcs are absolutely huge - bigger than some of the Chronicle and Citadel ogres! But they can all work in the same units if you like a bit of variety. If you're basing them on 25mm squares, though, some of the giant orcs need a bit of thought about ranking up.