At one time polys were bigger. My 230w poly is slightly bigger than my 245w mono and considerably bigger than the 220w mono. The trade off for a smaller footprint was less low light abilities.
A while back I remember reading that poly tech was still advancing and it seems footprint wise it has caught up with mono. I'm still waiting for the light cloudy conditions that will knock the mono down to a few amps to see how the new polys will do but so far they seem to have the other low light characteristics of the 230w poly so I'm hopeful.