Both my youngest brother and nephew have had this.

My brother is about to turn 18 and he has had no issues since he was born.

My nephew (who just turned 1) was diagnosed when in my sisters belly. They checked again at 30 something weeks and it was still the same. When he was born they did some ultrasounds on him and referred him to a specialist who put him on some medication (anti biotics perhaps?) which he will be on for another 6 months or so. If the other kidney is still enlarged they will look at operating, but at the moment everything seems fine and he is a perfectly gorgeous & healthy little guy.