Boys saw that I bought a big bucket of water in the school cafeteria and said, "Are you a buffalo?" What do you mean, (I'm a girl)

This sentence has no special meaning, just because I think you bought too much water, so I used an exaggerated tone.

Because most people buy small bottles of water, which are enough for their own temporary drinking, boys are a little surprised to see you buy a big bucket of water.

Moreover, boys speak loudly and don't know how to take care of others' faces. We usually use the word "buffalo" to describe people who drink too much water, so he says you are a buffalo.

There is no need to worry too much about this kind of thing, and there is nothing wrong with others. At best, they speak without thinking.