Have you ever seen them side by side, or anywhere near each other? Henderson is 6'1 and anywhere between 200 and 215 lbs depending on what division he's fighting in.
BJ is 5'8 or 5'9 and probably about 175. Just because he let himself get ridiculously fat back in the day doesn't mean he and Henderson are even remotely the same size.
Guys look the same size because they fight in weight divisions with other guys who are the same size.
Why am I even posting this?