I agree with Thani.. *cough*Kopa*cough*
In the Special Features of the DVD, the animator working on Mufasa did mention how he wanted Mufasa to be huge. He overdid some features because Mufasa was the king, the leader, and he needed to look and feel like the biggest, strongest lion of the pride.
Of course Simba would not be a huge bunch of muscles, living off bugs since cubhood, but I also think that he wasn’t finished growing in TLK. At the end of TLK he is a young adult. In SP he does seem to have grown, and his body looks bigger and "fuller." (He’s not fat though.)

Simba has matured into a fully grown adult, his new diet of meat giving him a bigger and stronger body. This also supports Kopa theories, as a lot of time seem to have passed between the end of TLK and the beginning of SP.