If the 5090 is 21,504 SMs and the 5080 is the full 10,752 SMs (a generous assumption that presumes no 5080 Super/Ti using the same die) that would still make the latter just half of the former (or 57% of the VRAM).
This could be related to US GPU laws against China - you can export a 5080 without restrictions but not a 5090.