It implies:
* Gluttony: The person ate much more than a normal or reasonable amount.
* Lack of restraint: They didn't control their appetite.
* Possible embarrassment: Their overeating might have been noticed and even caused them shame.
Example: "John made a pig of himself at the buffet, eating three plates of food!"
This idiom has a negative connotation, suggesting someone's behavior was excessive and potentially offensive.