Bettas need about 2 liters/1cm adult fish body, meaning that one betta would be pretty comfortable living in a 15 liter tank even. They live in little mud puddles in nature anyway so they're used to the small space (and jump to another puddle if the one they live in dries out...).
Cool. Who would think a fish would live in rice patties?
They live in rice paddies, the puddles there are far from small
In fact I have been told that male betta's can defend a territory size of up to one square metre, not really all that small in the end!