From the thumbnails I thought he was getting kicked in the balls over and over. While it's still been cringe-inducing, the actual location of the kicks was a pleasant surprise.
In context of the series it means she's fat because she's always pigging out on food, even when she was in the womb. It sounds like it's parodying an adage, but I may just be thinking of "born with a silver spoon in her mouth," which I'm pretty sure doesn't sound the same in Japanese if it even has a Japanese equivalent in the first place.