No one is denying it, but it is more water. If you don't have a large enough bucket to carry the amount of water needed for the water change it is a lot of trips back and forth and a lot of weight to carry if you do have the right sized bucket.
It's better for the fish to have clean water and room to swim and nice warm temperatures. Chemicals do build up faster in smaller tanks. Sometimes the owners may end up finding their own perfect middle ground for their betta--a tank size they can easily change(in order to change the water quickly before ammonia builds up) and a tank size that gives the betta all the room it needs. For some people, that is a 2.5. For others that's a 5. Some people that's a 10.
If the owner of the betta was not able to change a 10 gallon often and dreaded it enough that they began to put it off, the betta would suffer, because even in the large tank ammonia will end up building up and spiking. If that owner is able to keep a 2.5 gallon tank warm and clean and free of ammonia, well at least the fish does less suffering.
But for other people a 2.5 gallon tank will build up ammonia too quickly and they won't be able to keep up with the water changes. They don't mind changing lots of water as long as they have that extra buffer time to keep their fish healthy. A 5 or 10 gallon will likely be better for them.
If everyone could give their bettas a 5 or 10 gallon tank and could take care of them properly, that would be the best. But people are going to have different needs just like their fish. Some people just won't be able to care for a betta no matter what tank it's in because they just can't find the time in their life to clean it. We've all heard of those tanks that hadn't been changed in years and somehow had fish clinging to life in there. But some people might be able to find a perfect balance to keep the maintenance easy enough that they can keep their fish's needs fulfilled. And like I said, a balance. That doesn't mean that people should put their fish in .2 gallon tanks, but it means that maybe a slightly smaller 2.5 gallon tank would be better for their fish because their fish would end up seeing less ammonia.
Whatever leads to the healthiest fish. A large tank can lead to unhealthy fish, when the owner believes that them being in such a large tank means that they don't need any water changes anymore, or only changes them every 3 months or so.