My thoughts on why they can grow bigger in the wild, is they are growing at a natural pace. Captive raised animals are fed on "we" feel is the right prey size and frequency, most often what is most convenient for "us."
You can powerfeed a boa to make it larger faster, but you can also do the opposite and stunt a boas growth by not feeding it an adequate amount. Although no one wants to admit that they do either of these things intentionally. 
Also smaller boas are quite popular, so many breeders are trying to "manufacture" boas to stay under a certain size, whereas in the wild the larger, more robust boas from a litter are the one most likely to survive.
Amie