Is it obligatory to finish leftover food after one is full to avoid its consequences on the Day of Judgment, or is that considered extravagance?

You are not obligated to force yourselves to eat leftover food. Rather, moderation in spending, eating, and drinking is encouraged. If there is surplus food, it should be given to needy relatives and neighbors, or be given as charity, or to animals. One should not force themselves to eat it after being full, as this involves extravagance and harm to health.