frequent blow outs = too small doesn't diapers?

Question for the experienced mamas put there. My daughter is 7 weeks now and had a hard time gaining weight in the beginning. She's gaining just fine now but has always been on the smaller side - she weighs close to 9 lbs now and we still have her own newborn clothes and diapers. However, over the past week we've been dealing with frequent (once a day about) diaper blow outs, where this never used to be a problem before. Should I try moving her up to size one?