I am using the Billingham Photo Vests for two years now an are very satisfied with it.
It looks good, is strong and the design of the vest makes it nearly impossible to see whats inside the pockets
If you download the pdf from Billinghams site, you can read what size fits the best.
My experience is that it is better to chose one size that is larger then indicated.
as far as I know the size of the pockets do not increase noticeably with increased size of the vest.