The good news about leather is that it stretches like crazy. The bad news about leather is that it stretches like crazy.
Yes, leather can and will stretch while being sewed. I glue all the parts of leather seat covers to sew foam, especially the perimeter pieces. I have some sew foam that is only 1/8" thick which is what I use for the perimeter pieces ( the parts below the top of the seat cover). Doing this makes the parts much easier to sew together, and makes fabulous looking seat covers. This also prevents the perimeter pieces from stretching when you're installing the covers on the seat frames, and pads the seat frame so the leather doesn't rub against the metal.
This seat is a black leather Cadillac seat that was done that way.
No one lives forever, the trick is creating something that will.