DUBIOUS : His wife is a little bit too fat.
GOOD : His wife is slightly overweight.
DUBIOUS : He's got very fat since we last saw each other.
GOOD : He's put on a lot of weight since we last saw each other.
In western society being fat is considered to be unattractive. The word fat is common but it is not polite. To say the same thing in a less direct way, use words such as overweight (weighing more than is normal), plump (having a pleasantly rounded shape), chubby (to describe babies and children), stout (short and rather fat), or large / big : 'Large people sometimes have difficulty finding fashionable clothes to fit them.' Another way to avoid fat is to use a phrase such as put on weight or (have) a weight problem: 'He's worried about his weight problem.'