When it's time to move up in diaper size?

This might sound like a dumb question but my daughter is going to be 4 months on the 25th and she's currently wearing size 1 diapers. I guess my question is how do you know when it's time to move up in size. I know the weight is on the box but that doesn't seem to help for me right now. I feel like size 1 diapers fit her fine around the waist but are a bit snug on her chubby thighs. When I change her diaper she has little marks, is that normal or dose that mean time to move up in size?  I have tried a size 2 on her but they seems a bit too big and might leak. I want to stock up since Cosco has a sale going on with diapers but I don't want to buy size 2 if there still a bit to big but I don't want to buy size 1 if there's 192 diapers in the box and she might grow out of them within the next couple weeks. Lol help. #momproblems #diaperproblems