People forgot what Garnett was like when he first came in the league. He only recorded 6+ rebounds in his first season in like 28mins.
Nobody is saying Yi'll ever be as good as KG, and he probably won't. Yi isn't on KG's level when it comes to explosiveness and quickness. People are mainly comparing their game style. Yi has a nice jumper, great body control, similar build and good athleticism(a mixture between Theo Ratliff, Stromile Swift and someone else according to Del Harris). These are all that inspired people to draw that KG comparison. Sort of like Eddie Curry was called Baby Shaq. Yi is still raw so don't count on him dominate games yet but he does have nice potential.