I've seen this "20 inch max." quote pop up a few times and it's unfortunately just a problem with adjusting to a new taxonomy.
Both the Amazon and the Orinoco have two common species of Pseudoplatystoma, one with irregular stripes and one with straighter (ish) stripes. For quite some time it was thought that the two species were shared between the river systems, so both P. tigrinum, and P. fasciatum were said to be found in the Amazon and Orinoco. In 2007, though, a pair of ichthyologists decided that because the two basins are more or less isolated, the species in each basin are probably different, so the two Pseudoplatystoma were given new names. The one that looks like tigrinum was called P. metaense and the one that looks like fasciatum was called P. orinocoense. In the formal description, most of the specimens examined were relatively small, simply because they're easier to work with and transport, etc.
So, for someone gathering data for fishbase, it would appear that P. orinocoense was "discovered" in 2007 when it was first described, but really, it was just called P. fasciatum before that. Most of the Orinoco is in Venezuela, which has been rife with turmoil for the last 10+ years, so very few comprehensive fish surveys have been done...but from before that, we have good evidence that Orinoco fish reached generally the same sizes as Amazonian fish.
In 2003, for example, Barbarino Duque and Winemiller published the paper "Dietary segregation among large catfishes of the Apure and Arauca Rivers, Venezuela" about Orinoco River catfishes and in it they note a 39 inch, 28 lb P. fasciatum, which would now be P. orinocoense....
www.aqua-imports.com
www.planetcatfish.com
