You sure they aren't just mislabeling? I had to buy a rat small for my 1000g girl from Petco a couple weeks ago because hubby forgot to get it at the locally run reptile place and they were closed by the time I got off of work, and the rats Petco was selling as "mediums" were definitely what everyone else sells as smalls. It was sevens bucks too, double what I usually paid I was so mad
I'd weigh the rats you're trying to feed just to double check, and get an updated weight n your girl to boot. You want to shoot for prey size that's about 10-15% of her weight.