If the contents of your wallet are busting out at the seams, it's probably time to clean out what's not necessary. And most of the things you're carrying with you probably aren't necessary.
It's not the fat stack of greenbacks we're talking about (although, keeping a small amount of cash in your wallet in case of an emergency or lunch at your favorite food truck isn't a bad idea); it's all those pieces of paper and cards that give away your personally identifiable information that you shouldn't be carrying around.
Remove the old purchase receipts, deposit slip stubs and all those appointment reminder cards -- and although it may seem convenient carry a blank check with you, don't. Then, cut the cards: Carry with you only the identification you need. Never carry your social security card or passport, and remove any credit or debit cards you won't be using. Keep these -- and any other card that includes your personal information and aren't a daily necessity -- safely at home, and keep photocopies of all your cards (don't forget, front and back) so you have account information and important phone numbers handy in case there is a problem.
Other things that shouldn't be stashed in your wallet include PIN reminders, as well as your spare house key (if a thief gets your wallet, they get your driver's license -- which has your address on it).