I think full sized headphones typically produce higher quality sound. But such a general statement will always have exceptions. I've used IEMs for years but recently bought some full sized headphones and they are definitely more detailed.
Both types have strengths and weaknesses and they were designed for different purposes, much like laptops and desktops.
I have a pair of Shure SE215 IEM and a pair of Sennheiser HD449, full size, they costed me about the same, and I couldn't rate any of those higher than the other in sound quality. Both lacks in some areas, both shine in some others at their price point,
Aside from that particular example, I'm on the same boat as Kurious, usually full size headphones have more raw material to work with, more air, more space and thus, it produces a higher detailed sound at lower price points.
In general, a full sized open headphone will sound better than anything else for the price. There are of course exceptions. Also, full sized headphones tend to last longer than IEM's. IEM's can't be beat for portability and noise isolation, though.