A.L.
Hi M.
there is an interesting article in the nytime regarding bedwetting.. http://www.nytimes.com/2010/01/12/health/12klas.html?ref=...
albeit, you aren't asking about that perse, but it's related in the sense that the article discusses bladder sizes and how in some cases, some children just have a smaller bladder (nothing unusual and quite normal) it's worth a read...
best of luck