Where Do We Draw The Line?

I was recently reading a blogpost where the author got prosecuted by hus faithful readers for not differentiating between big, fat, overweight and thick.. :s Very very confusing i must say! I didn't realise there was a difference! Thick maybe, example, Kim K, just look at that body! Now she is THICK as hell but nowhere being fat or overweight.

As for the others I'm at a loss of words, and I like to ramble.. Surely being fat implies that you are overweight probably because of all that extra FAT hanging off your bones? And being fat implies that you're big, again all that fat takes extra room to store, the more the baggage the bigger the suitcase right?? Which of course in a roundabout way means big also equals overweight!

I recognise people have varying opinions on this, someone please educate me??? What if any is the difference between being big, overweight and fat?