I think it has more to do with the late transfer than the weight.
Look at his
sophomore film and compare it to his
junior tape. Last year, he was asked to run short and intermediate routes exclusively. Hightower's two top receivers both caught a majority of their passes under 10 yards. That's a scheme choice to spread and take quick yards. Maybe his late arrival made them just plug him into the slot there? Maybe he wasn't in the best of shape? Maybe his quarterback couldn't deliver on time down field?