In general, people do not like to be called fat. But some ways to say 'fat' are less rude than others.
fat is a very direct word. You might use it about yourself but it will usually cause offence if you use it about someone else
• I'm so fat at the moment!
overweight is a more polite way to say that someone is fatter than they usually are or than they should be
• She is a little overweight.
obese is a word used especially by doctors to describe people who are very fat, in a way that is bad for their health.
chubby is a more informal word and is used especially of children or of rounded body parts such as cheeks or knees.
plump means fat and rounded in a pleasant way
• a plump, motherly woman
big and well-built are fairly polite ways to describe someone with a large, strong, or fat body
• For big men like him, air travel can be uncomfortable.
SOURCE: Longman Dictionary Online
