You really gotta take into account the average fish doesn't hit its potential size, or it's max age unless kept in pristine conditions with the best of experience and knowledge of the caretaker. I am reading the MAX size is 5"-6" with the above taken into account as being great conditions. Most are not perfect.
Still better than the guy who sold me my 10gal tank. Dropped it off and ran. I come to find 2 Common Plecos in it... one 5" and one 6". My female is 8" now (bought her a 29g). Guess who has to buy a suitable tank, JUST FOR HER, again? Urgh.