With a big check in luggage, I may travel with shoe trees. Most times, socks rolled into balls suffice as a makeshift.
And as far as gym shoes go, New Balance Minimus Zero Road shoes are a godsend. They pack very well, and they look pretty good as casual shoes too (at least I think the first gen range is). Of course, one may need to get used to forefoot strikes, in the first place.
I also have the same habit of overpacking, but it is mostly limited to additional underwear. And maybe an extra t-shirt. And maybe an extra pair of pants.