I think you could measure bioload.
Set up a tank, add the fish in question and measure how much the nitrate level went up each week.
You could end up with a standard unit, that says 1ppm, per gallon per week is 1 "Bio"
So if your Oscar raised the Nitrate level by 10ppm in a 50gal tank, then it's 500 "bios"
If 10 guppies in the same tank took 5 weeks to raise the level by 10, then they would rate 10 "bios" each.
Size of the fish of course has an effect, but their diet and metabolism will have a big effect too. I have an Oscar and Pleco of similar size, the pleco makes more visible waste, but it's mostly recycled algae and cucumber. The Oscar is pooping out recycled fish etc
But I guess, as the article is pointing out, it's the amount of food going into the tank. "Messy" fish like a healthy young Oscar eat more than an adult Betta of the same size. But generally more and/or bigger fish eat more, so produce more waste.
Ian