Although previous rumors revealed the existence of two telephoto cameras, the latest Samsung Galaxy S21 Ultra leak also uncovers their technical details. According to the popular leakster Ice universe, this optical zoom duo consists of two 10 MP cameras with a pixel size of 1.22 microns, of which one sports 3x zoom, and the other has a 10x magnification factor.
https://www.notebookcheck.net/New-Samsung-Galaxy-S21-Ultra-leak-mentions-a-10x-optical-zoom-lens.504199.0.html
Well, S20U's 4x was supposed to be 1/2.55"(which Sony claims to be 1/2"), IDK about the previous 3x etc. though, and down to 2x we have the 1/2.3" Xiaomi used, at f/2, no less.