Maybe try a different brand? I've found pampers most leaky tbh. Or have a look at reusable nappies. I am mostly using those and get a much better fit on my baby. Disposables were leaking quite a lot.
We had the exact same problem, no matter what brand or size we used it was the same. I found the only trick that worked; put a few cotton pads between the waistband and their back, it slightly fills the gap where they lack back fat and helps to absorb any leakage. We did this for a week and now he’s fattened up abit and filling the waistband out more we don’t have any problems. But this definitely helped reduce the amount of leaking! I used the large cotton ovals, didn’t irritate his skin and just put 2 side by side at his lower back xx
Always flare out the folds in the nappy and make a little pocket at the back by folding the top of the back of the nappy down.