Japanese Sentence "You are stupid/You are so stupid" difference?

Why is there such a big difference between saying "Anata wa baka" (You are stupid) and "Kimi wa jitsuni baka da naa" (You are so stupid)? I have been studying Japanese for only a few months so I am not very familiar with certain ways the words are used. Also, I understand that this can be very rude, so it's not something I would go around saying like a "Japannabe", I am just curious why it is said so differently (if I got it right).