How do do you stop pee from leaving the diaper?

Title says it all.

At 2 weeks our son is very often waking up with a pee stain under him. Doesn't matter the position he sleeps in, the type of milk (breast or artificial), the size of diapers (we downsized from a size 2 because of this problem and with a size 1 he still does), brand of diapers...

Tbh he eats quite a lot.

The pee almost always comes out from the side or back of the diaper, never the front. Are we doing something wrong? We never had this problem in the hospital.... Are there any tips to stop this from happening?

Edit: thanks y'all! We tried a combo of proposed solutions, amongst those maybe the most impactful were tightening the diaper better and fluffing out those leg flaps completely.