Why do fish need so much space in an aquarium?

I bought a 30 litre aquarium and went to the store to buy 2 mollies to put in it after cycling it for 1 months, but the man there said nope, 115 litres or more are the only aquariums good enough for them. The only fish he would sell me for 25 litres was a single Betta. Why do fish need so much space? It seems... show more I bought a 30 litre aquarium and went to the store to buy 2 mollies to put in it after cycling it for 1 months, but the man there said nope, 115 litres or more are the only aquariums good enough for them. The only fish he would sell me for 25 litres was a single Betta.

Why do fish need so much space?
It seems strange that small fish need more space than mice or rabbits do.

I am not at all saying fish should not be treated well, I just bought a 125 l aquarium because I want my pets to be happy and healthy, but I just find it strange that a petshop will sell me a 2x4 metres rabbitcage and say that is good, but the 3 centimeter long fish needs more space pr. centimeter/liter than any of the other animals Is there a reason for this?
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