Small Format Professionalism, excellent article by Ctein about how bigger is not always that much better, and how small can be better than you think.
It's [the idea] that small format digital cameras are not suited to really serious professional photography, certainly not the kind that results in portfolio prints. Oh yes, maybe screenshots. But the hard stuff? You need a big sensor for that!
Nonsense. Balderdash. Hogwash. Bunkum.
Why can't we stamp this one out? Honestly and truly, real-world results just don't support it. They haven't for years. Not even by highly demanding aesthetic standards.
There is also this interesting discussion.
I seem to recall Olympus saying, back when their first 4/3 DSLRs came out, they expected 4/3 sensors to some day equal medium format film.
Seems to me they are pretty much there. My prints from m4/3 pretty well beat anything I have from 645 medium format and match all but my best from 6x7 cm.