If you ever create a system where the number of users is “group.members - 1” everywhere in the code, I’d be very disappointed in you and deny that PR.
On another note; I doubt WhatsApp are so concerned with performance they are actually limiting the number of group members by the data type.
I hadn’t thought about it on their server side tbf. But the more i think about it maybe there are other compounding reasons to keep group sizes small, such as the exponential number of links in a growing network and such. But, that is all beyond my knowledge area.