There's a thing called heat warping.
The concept itself is pretty much straightforward:
When you heat metal, it expands. Cool it down, and it shrinks.
If the temperature gets high enough (especially w/ metals that have good thermal conductivity), it could warp it.
I've seen this happen while using my friend's HP's Gen1 Envy 15 (prone to heat issues), in which his actually slightly warped after a several-hour intense gaming session.
However, this isn't a problem with the MBP's (i've actually tried to get it to heat warp).
If it's not what Ramphex said about the swelling battery, it's probably just your head and the heat.🙂