Where are you getting those stats? A quick google search (literally just "bieber") lists Bieber as 5'9.
And it's obvious he hasn't gotten CLL due to his stamina for dancing and all the shows.
He could have, instead, received all kinds of HGH injections, IGF-1 level increasing medicine, and all paired with AIs, possibly since an earlier age. All which also wouldn't normally be possible by non-rich parents - much less known about by the parents, who wouldn't (ironically) be as concerned for his image.