They’ve shipped with both 64GB and 128GB boot drives. I don’t think anyone has come close to crossing 64GB. It was just hard to source 64GB components that met the performance requirements at some point, so we went to 128GB.
Again, another reason why the “technical specs” are irrelevant. The important spec is the performance of the SSD for random reads/writes, yet that’s not what anyone looks for nor do others advertise it. The focus is incorrectly placed on capacity.