The issue isn't storing each individual ID, it's all of the networking operations that are done and total things that are stored/cached per user in each chat. All of those things are handled and stored as efficiently as possible. Sure they could set it to any number, but 256 is a nice round one when considering everything that is happening and the use cases involved. They have user research data and probably see that 128 is too close to a group size that happens with some regularity, but group sizes very rarely get close to 256, and 512 is right out.