Some bettas really do prefer smaller tanks. While I am not saying to go out and get a .2 gallon (lol can you imagine), 2 gallons, though seemingly small, can be perfect for a betta. Obviously, more water changes may be required, but size-wise it is fine. I think 2.5 gallons and up is a good general rule, as most bettas would appreciate the larger size, but going a bit below to 2 gallons may be the only way to keep a betta happy. My betta loves his 10g but I have heard of bettas here that got larger tanks and not like them at all, even allowing for adjustment time. They just weren't happy outside of their little "area". I guess they are similar to humans, some people like big mansions, some like average houses, and others prefer a small cabin. Lol, my analogies.
Also, at target they sell these glass jars to hold flour and sugar which would also work as a tank. They are also about two gallons.