Guuuurlll this child is already in three month clothing but rompers still don't fit her chunky…

Guuuurlll this child is already in three month clothing but rompers still don't fit her chunky little thighs and 3-6 is way too big on the rest of her. Most of her summer clothes are rompers but her thighs are getting a little squished by the elastic. Anyone else's babys thighs too big for their clothes? Is there any remedy? Ive tried pants but she gets heat rash even with the air on if she's wearing those. She's got a few pairs of shorts but the waistband on those is too tight in 3month and they'll fall off of her in 3-6. My son was never even a little chunky, we called him a noodle boy when he was a baby. So this is my first baby that's got some chunk to them and I'm not sure what to do about the clothing situation.