Lets assume 2 boars. 1 food = 10 minutes of breedable state for each animal. Through valheim gender magic both animals can, and often do, end up pregnant at the same time. For boars the pregnancy duration is 1 minute. After they give birth they can immediately breed and get pregnant again. You can easily get 6+ piglets for 2 food to 2 boars. That is for 2 pigs.

The amount of piglets you can get per minute goes up even further with more pigs because they can reach pregnant state faster from multiple partners. bowchickawowow

Edit: this isn't new. this has been in the game since at least Hearth&Home update if not sooner.