This is known as macroskelia and is due to the lack of test in the system. Macroskelia normally occurs today with chromosone diseases, i.e Klinefelter Syndrome. Macroskelia only occured in pre-puberty castration among eunuchs.
As long as the testes function, a man's body will continue to produce testosterone as well as sperm and he will still (theoretically) be capable of fathering children. To stop this castration was employed removing the testes completely. If this was done after puberty the eunuch would stilll retain masculine features.
Castrati on the other hand had their testicles removed before puberty, hence the castrated boy will retain many of his childlike qualities into adulthood including the inability to get a full erection, and a lack of body and facial hair. He will also have trouble putting on muscle mass.
Castration--not a route I'd consider for bulking up;-D