Does your wallet look like this?
If it does, do you actually sit with it in your back pocket?
I do just about everything I can to have the smallest wallet possible and I still find myself taking it out on long car drives, sitting in the office, etc. -- have you ever seen someone with a wallet like the one above actually sitting tilted? It usually happens on a non-cushioned chair, but you can actually see a guy sitting cockeyed because his wallet it offsetting his equilibrium.
I'm sure that there are studies showing how fat wallets can have a negative impact on your spine, etc., but really this is just common sense: if you were sitting with one side of your butt on a brick and another not on a brick, and you had the ability to massively reduce the size of or eliminate the brick, wouldn't you do it?
Outside of the sitting issue and potential health problems, fat wallets just don't look good; there is absolutely nothing impressive about carrying around an enormous wallet.
Just my $0.02, of course, you may disagree with me.
PS -- do yourself a favor and visit Just One Club Card to consolidate your club cards as a start. Do you really go to Petco often enough to carry the card in your wallet? I didn't think so.
Picture from mikeying88.