You're getting downvoted because it's probably not the reason that the OP was talking about, but sitting on a large wallet - full of cash, receipts or just junk - is VERY bad for your back. And most people don't even realise the damage it's causing. Also, it's insidious in the way that a wallet starts out thin and flat, then slowly gathers remnants of daily life. Getting larger and bulkier by the day.
I'm being totally serious here. If you carry a wallet around in the back pocket of your trousers/pants/jeans whatever, and it's full of junk - even if it's not - and you have chronic back pain, stop doing that! Put it in your front pocket or jacket. Anywhere. I assure you, it's very likely to be causing problems for your lower back.
As a test, just switch the wallet to the other side and sit down. It will either feel great or hurt immediately.
I never been able to put my wallet in my back pocket. It's so uncomfortable, especially when sitting down like you said. I can't understand how other people put up with it. I always carry it in my side pocket instead. It looks weird but at least I can sit comfortably.
