This is probably a stupid question but how do you know you need to go up a size in diapers???

R will be 2 months on Saturday and currently is in size 1. I just ordered a big box from Amazon but now I am wondering if we should go up once we go through the ones we have opened. She is lucky and inherited her Mom's chunky thighs so the diapers are tight on her legs but they don't seem uncomfortable to her. The waist still seems big on her. The tabs are still touching. We go on Tuesday for her 2 month appointment but I am guessing she is 11lbs since she was 9.14 at her one month, which was 5 weeks.

I don't want to open the new box unless we will use the whole 234 diaper.