As members you get 1 extra bank spaces for every new quest, item or random event you get. Its just that its a waste of time to give 3 or 4 bank spaces every week to you. Thats why instead Jagex gives you 50 each and every other month. As for f2p, we get a ranging update and a few new random events. Yet still we haven't got at least 1 new bank spot. This is one of the reasons why me and many other f2pers want atleast one more spot.