I started witha 35 hex, then a 55, now the 180. I don't think tank size mattered with mine. He was going to grow and nobody was going to stop him!
What/how ofter do you feed? Plecos need a lot of food. This may have something to do with growth rate. I feed a combo of Hikari algae and sinking wafers am and pm, and every other day he gets fresh veggies for dinner.