I second everything Bill said above. Additionally, I find the sound of a smaller-bodied guitar compliments many of the songs I sing better than that of a dreadnaught. When I play a dreadnaught while singing, I feel a tension, almost a competition, between my singing voice and guitar. If I play my OM or 00 it feels like my voice is leading the way and the guitar is truly accompanying it, not competing with it.

That's not to say it's necessarilly bad to have that vocal/instrument tension going on. Some styles, such as bluegrass, demand it, but it's nice to have an instrument that naturally provides more complimentary accompaniment for some songs.