I think that a few people are either mis identifying the pleco they have seen or over estimating the size of the one they saw or had. THis species does not get much bigger than 12" perhaps 14" to 16" on the outside but that would be far and few between.
I am not basing this opinion on only one example or experiance I have had with this species (as many people do) I kept one of my own for a very long time but I also have them in service accounts and have sold several of them to customers. I also did a quick search on line and I think the bifggest size I saw was up to 16".
There are several Hypostomus sp that may look similar and do get really huge.