Another good limitation is the one found in the Ethshar series by Lawrence Watt Evans. In that series you could fit a military unit's entire supply train into one of those bags but items had to be removed in order - basically a LIFI (last in, first out) system.
For example, if you used it to hold (inserted in this order) your car keys, a water bottle, your wallet, a gun, a sword, a package of cough drops, then you couldn't get access to your wallet without removing the cough drops, sword, then gun.
Richard