Baby’s too big for diapers?! What can I use??

So my daughter is 21 months and she’s been in size 6 for a bit. But now they’re seeming way too small. They fit almost like a thong 🤦🏽‍♀️ and she’s constantly spilling out every night. If I don’t change it 3 times a night it spills out. So I’m pretty sure size 6 is too small as this has been the indicator that I’ve needed to switch sizes every time. The only thing is, size 6 is the biggest size? What can I use then? She isn’t potty trained yet but we’re working on it slowly. She hasn’t gotten it down yet so she absolutely needs diapers for sleeping. Anyone know what I can use?