It's more the time constraint angle. He admitted he was expected to put on 30 pounds in 3 months. Also he was trimmed down before he started bulking for bane.
A lot of these guys have good bases and very well could get to these physiques naturally but the very fact they often have only a few months to put on so much weight and with all the pressures that come with a multi-million dollar pictures i wouldn't be surprised at all if they went to other methods to gain on the muscle.
Jimmy kimmel pretty much accused wahlberg of juicing in a recent interview and you could tell wahlberg looked really uncomfortable. I think it's a hollywood dirty secret.