"It's very fat" 可以说成 "It's overweight" 或 "It's plump"。
"Overweight" 是由 "over" 和 "weight" 组成的，意思是 "超过标准体重"。
"Plump" 来自古英语 "plumpen"，意思是 "丰满的，丰胸的"。
例句：
"She's overweight, but she's still very healthy." (她体重超标，但仍然很健康。)
"He's a pttle plump, but he's very strong." (他有点胖，但很强壮。)
"The cat is very plump." (这只猫很胖。)