Overnites leaking

My babe is 9.5 months old. He is a 31.5 inches long and over 25lbs. He wears a size 5 Sams Club diapers during the day and size 6 huggies overnites. Both diapers still fits great visually and we actually recently just started using these sizes.

Anyways, he sleeps 10-12hrs at night and the past 3 nights, he had leaks, all the way through his sleep sack.

I honestly think he is not ready to size up but interested on why this keeps happening when we just recently sized up. Huggies overnites also doesn't have a size 7 so I will have to figure something out. (Pampers don't work for him)

Any suggestions?