Of course, it varies by individual, but in general, rebounding usually translates very well from one level to the next. Much more so than scoring, for example.
Of course, size helps, but rebounding is a lot about effort and players that "get after it" at one level usually bring that same intensity to the next level. If you watch Collins play, he pays with a lot of intensity and effort.
Regarding your question about 7-footers in college, Collins was a teammate of one of the biggest men in all of the NCAA, 7'1", 304 lb. Przemek Karnoski. They played in the same system against the same competition. Collins had a TRB% of 18.3 and Karnowski was at 13.6. That makes Collins an above average rebounder for his size/position and Karnowski a below average rebounder for his size position.
BNM