Growth rate variation.

[Ginevive]

My male BCI is about 2 yrs. old, at roughly 5' and a good weight (haven't been able to weigh him recently.) Now, my man's uncle has a female BCI
that he got the year we got ours, and she is HUGE.. I mean, she has a good foot on ours (or maybe a little more!) and is much, much girthier. I am betting this is just an obvious male vs. female growth rate, but he even feeds his less often (once every 2 wks, while I feed weekly.)
So I guess this could be a gender issue, as well as possibly a genetic thing? They are kept in similarly furnished and heated cages.

[Pork Chops N' Corn Bread]

It could have to do with many variations, These are just things I came up with off the top of my head from Experience ect.....

Feeding(males require less food than females GENERALLY!! The older they are, the less food they need. Your male is likely younger than your uncles. A male at 5' would do fine on a every 2 weeks feeding schedule IMO)

Sex, of course females will get larger, girthier, and weigh more than males

Locality, Your male may be a completely differnt locality than your uncles female. For example, your male could be Suri(which get 8'+ usually either sex) and his female could be a hogg island (females rarely getting over 6') and much older

Age, self explanitory.

Hope this helps you out a bit