But really, 10 gallon is the minimum you can maintain an adult leo in. Sure, 15 is better (though actually a 15 isn't much bigger at all), but you can maintain a healthy, happy leo in a 10 gallon. I have heard of people keeping 2 or 3 adults in them too...I wouldn't do it, and I wouldn't suggest doing it though. That's not really a good way to keep them. It's just the same as how people say 40 gallon breeder is the minimum for a bearded dragon or a uromastyx. I have both and feel that a 40 is too small (that's why I'm building new bigger cages for them). But just the same, i have seen people keeping pairs and trios in them. I once saw a tank at a local reptile shop with 5 or 6 adult bearded dragons in a 40 gallon breeder, and I've seen the same with uromastyx....I actually got my beardie there and he was just filled with parasites, so that's not really a good place to go anyways.
The 10 gallon tank thing for leos (like the 40 gallon with beardies and uros) is sort of just a rule as to what the minimum you need to provide is. Some people break this rule, others go way above and beyond, but I don't think you can say that you can't maintain a healthy animal in tanks those sizes. If not then there's something wrong with the other aspects of husbandry.