Typically you see that the average male can add 1-2 pounds of muscle per month, so based on that statement alone, Hubbard being able to go from 205 to 265 seems unlikely.
But there are several other issues to consider. The average man is about 5'10" tall. Hubbard is 6'5". The average man has an average skeletal frame. Hubbard is much bigger. The average man isn't a genetic freak. Hubbard seems to be. The average man isn't between 18 to 20 years old, when your body is producing by far the most testosterone of your life.
The average man doesn't have at his disposal the type of training, coaching, schedule, and nutrition available to him as a Division 1 football program.
As a lacrosse player in highschool, Hubbard wasn't likely conducting the kind of training that would put on bulk. Actually, all the running he did in lacrosse, tends to prohibit large muscular growth.
And finally he was closer to 220+ than 205 when he actually arrived at OSU. So in the final analysis, it is very possible for Hubbard to add 40-50 lbs (about 2 pounds per month) over the course of two years.