Two readings to keep apart:
胖 (4th tone) is the everyday adjective "fat / plump" - opposite of
瘦 - and the reading in nearly every compound (
肥胖,
胖子,
发胖). A literary
胖 (2nd tone, "at ease / contented") survives only in the chengyu
心宽体胖 - never use pán in modern speech.