I'd laugh if someone told me I was fat. If I was though, they'd have to understand that losing weight isn't something you just press a button and do. I think it's dumb to tell somebody they're fat personally. I'm pretty sure the extremely large people I see in the stores are aware that they're overweight, but for the most part getting fat doesn't happen over night and neither does losing weight.
Think of this scenario. If I gain 1 pound a month, that'll go unnoticed. If I gain a pound a month for 20 years however, I'll have gained 240 pounds, so being fat is easier than people think, especially with all the junk that fills our grocery stores and the media giving false nutrition advice.