We've had this issue in the past as well. My son is 2 years old (27m) and weighs just about 30 lbs. We've been using the 2t-3t size and I notice that he gets scratches where the plastic easy open sides are sometimes. I'm assuming maybe we just need to go up a size?
I am using pull ups hecause they fit my 2 year old better than easy ups. The problem with them though, the velcro easy open sides scratch my sons hips. On the top of each hip he has several scratches, some deeper than others, from the plastic.