And look at Brandeis Voicemale and the Berkeley Octet. Among the consistently best and tightest, and never taking on more than 8 people.
Obviously, you can take the best of the best and end up with a small group, or you can take the best and the mediocre and end up with a bigger group in which the mediocrity is equated for because parts are doubled. Seems like the trend as of late has developed into the latter.
My theory is that because so many groups have arisen as of late, all too often from the embittered few who don't make the existing groups, the audition pool is diulting. This is also due to groups starting at schools where there is no a cappella yet and therefore interest is scant since audience following is yet to be built and therefore being in said groups is not yet "cool". Because of this, groups have had no choice but to go big because they need parts doubled because of the weaker singers, tonal centers distorted by an increased number of voices, and added volume because of those who sing without balls.
That's where I think it's started, but once it does, it doesn't matter. Once you've got 15 people in a group, it becomes the standard, and you feel naked with 11 or 12 guys. And you totally see that happening today. I guess there's no turning back now.
The Binghamton Crosbys have been 15 guys since the mid-80's. Maybe it was because of the reasons I mentioned above back when they started up, but that size has been the norm since then. Ahead of the curve, I guess. ;) One difference is, though, that we work under the mentality of, "we've got 15 guys, we sure as hell had better be loud, exciting and energetic". Whereas I see groups of 18 and 19 that sound like 10.
Personally, I'd love to see more groups not afraid to go smaller and be more selective. I think any group could stand to benefit. If your impetus for a big group is that you need them for your 13-part arrangements, that's understandable, but I'd say you might want to think about honing your arranging skills a little bit.