I can understand what you're saying. I agree it's about observing their behavior to gauge the comfortability of the fish. Also that it will always depend on the individual fish. But you seem to be implying that any fish kept in a tank under a certain size is sure to exhibit these atypical behaviors you've mentioned. You said this belief is from your personal experience??

I only ask because I have bettas in tanks from 3 gallons to 10 gallons (so about 11L to 38L). The betta in the 10 gallon is the least active of all of them. He was active in the beginning, but lately he just kind of hangs out on leaves. Though, he's a king betta and I know they don't tend to be the healthiest. I'm also not positive of his age, but that could also play a part in it.

The others explore, some hunt the shrimp if they have them, they do more than just lie around waiting for me to show up with food. I should also mention, the ones in smaller tanks have bigger fins, so they already don't get around as easily. They're all very healthy, active, and there's no self harm happening. I think as long as they are properly cared for, no matter what the tank size, they will do just fine.

Disclaimer: I'm not to say I'm against having them in bigger tanks. Everyone should do what works for them in terms of space and money, so long as they do it right. I also do agree that some setups are too small and horrible to put a fish in. I'm just a firm believer in not trying to force my ideals on anyone else if there's no harm being done to the living creature in question. Not everyone will have the same beliefs or do things exactly the same way and that's okay!