Some guys simply don't gain weight, their body doesn't do it no matter what they do, Yuri didn't gain much weight in the pros either.
The didn't work part though is right. He like Cormani was so much better than guys in high school that he could dominate without working hard, he continued the path in Boulder. As a result his technique, his knowledge of the game didn't improved in a way that showed on the field.
When he got to the NFL and managed to make a roster he got the message. "Work like a pro, study, learn, practice hard, get better, or you can be back on the street without a job in the time it takes us to fax paperwork to the league office.
He finally listened and had a solid career but I wonder how much better his career could have been had he worked harder in college and gone into the pros better prepared.