Hi, I had the same problem due to a previously broken toe one did not fit great, and stretched mine out half a size. This is a trusted method I use a lot on leather. It only works on real leather with not to much stitching. So on the quilted flats it would be harder.
1) take a pair of ankle socks and wet the toe part, press between towels to remove excess water.
2) put them on upside down, so any seams are on the bottom of your feet, the heel part rest on your ankle now.
3) if you need to stretch a whole size, flip the heel part back over your toes, so they are covered with two layers of fabric. No need to wet them again.
4) Put on the flats you want to expand.
5) Blow dry your feet on a hot setting. You should hold the blow dryer pretty close to the leather, leaving a few cm. Keep moving the dryer over the shoe to all the parts you want stretched.
6) Check after a few minutes, because this can go really fast!
7) when satisfied, remove moist socks and replace with dry ones, again on or two layers. Wear those for a few hours.
Your shoes will now have molded to your feet.