I'd also worry about siblings always being in the same classes. Depends on the siblings, of course, but I had one year where my sister and I had quite a few shared lessons, and I felt really hemmed in somehow, like I couldn't ever be 'me' (I tended to view school as a bit of a chance to get away from some of the pressures at home). I don't think it would have been good for me to be with siblings long-term, either, because we were competitive and ended up conforming to certain 'roles'.
I also appreciated the wider choices of friends and activities at the larger schools, and they felt a bit less intense, too, as I often felt scrutinised and sometimes just wanted the chance to relax a bit, which a small school might have made harder. We also mixed the classes up each year, so always seemed to be the chance for new friends, re-inventing yourself, changing habits/patterns/ruts that you get in to with other children.
I was academically advanced as well, and that was probably easier to deal with in a larger school where there were more similar peers. And a huge library, which was fantastic for me!
But for the right children, a small school could be great- think it depends so much on the school, peer groups, etc. If it works, wonderful. It's when there are problems that there is less wiggle room for changing things.