If Jagex thought about it thoroughly as a business proposition, if they were to increase the bank size of f2p accounts it would lead more people to becoming members and in effect providing revenue for jagex itself. Why? They might ask, well think about it. If you have more space to store your items, you have more space to hold items that cannot be used unless you become a member, also new item combinations for members will become apparent, leading some to becoming a member. Its all in persuasion, and the way for jagex to increase their revenue by getting more and more f2p players to become a member would be to persuade them to increase their bank size even further to unlock new item combinations etc. at its current size, the bank does not fully utilise this persuasion, as it does not allow enough room for f2p players to work out new item combinations or save items up in order to become a member. So Jagex, increase your revenue, you know it will work, so why not do it soon.