Some fat people don't mind being called 'chubby'. It doesn't make calling them that acceptable. If I'm walking off the street and some random guy calling another random guy 'chubby' and they both laugh, I still get a bad feeling about it.

It's like swearing. It might be extremely funny sometimes (specially when Samuel L. Jackson says them!), but still, swearing is bad karma; makes the environment less friendly (my English is not well, and I can't express what I want to say very well).