I think you are. While the number of teeth engaging the ring is constant during rotation, the chain tension is not constant. Sheldon and rykoala are just saying that the change in tension is negligible and therefore not a problem for fixed gear riding.

My question is the following: will a large (high 50s-tooth) biopace ring have a higher variance in tension than a small (low 40s-tooth) biopace ring?

I realize that both rings should be perfectly fine for fixed riding; I'm just wondering if there are any significant differences besides the number of teeth. For example, besides tension variance, would a large biopace ring theoretically provide a larger performance gain relative to a small biopace ring?