Assuming unchanged density of tissues, mass is proportional to volume and strength is proportional to muscle/bone cross section. According to the square-cube la

Assuming unchanged density of tissues, mass is proportional to volume and strength is proportional to muscle/bone cross section. According to the square-cube law[1], to keep the same weight to strength ratio under ≃20% higher gravity, body linear dimensions should be down-scaled by 1/1.2 ≃ 83%. So I think not many generations would be necessary, given individuals with 83% the average size or smaller are a common occurrence. The trait small body size should just become more common. [1] https://en.wikipedia.org/wiki/Square%E2%80%93cube_law https://en.wikipedia.org/wiki/Square%E2%80%93cube_law