6 MONTHS 52 LB??!?!?

From what I have read staffordshire terriors and american pit bull terriers only get up to 70 at full grown.... then why the heck is my guy 52 lb at 6 months?!??! He is new to us, had him a few weeks and do not know really anything about his past. I was told he's a staff but I'm just wondering if maybe he is mixed with something or if he is over weight... he doesnt look fat at all.... maybe he is just huge???? I feed him twice daily. I just dont know... any words of advice??