Inventory space is a problem in all games. Either you can only have 2 guns, or 10 slots or 100. Most F2P games take advantage of this space issue and charge you real money to increase the size. It’s obviously a balancing issue where the dev needs to have enough space for moderate use while not making it so big that their databases crash from billions of items stored.
When WoW started, you had 5 bag slots with a maximum of 16 per slot. Banks gave you 32, plus 6 bag slots for a rather large amount of gold. Today, you can get bags with 26-32 slots, which pretty much doubles the storage that existed previously. RIFT also offers multiple bag sizes but they’re still in Vanilla mode, so we haven’t seen the explosion, if at all. EvE has this problem too, but it’s combated by increasing your ship storage size (which is offset by reducing offense/defense).
A possible solution for the themepark crowd is the exchange and trade-up. Where each WoW expansion typically adds 2-10 new tiers of commodities – and MoP has added a stupid amount with cooking – you really are stuck with each character holding on to items, then mailing them across to players who can use them. What I suggest is a shared commodities bag. When I’m out and about and collect a piece of Turtle meat, who says my Shaman needs it? Perhaps my Rogue is the cook. If I was able to put items into an exchange pile, that all characters had access to, I would be saving on space. Many games do this today in the Action RPG sphere – D3 and Torchlight 2 shared stashes come to mind. Heck, WoW and Rift and all of them do it with a Guild Bank already. People have been forming 2 man guilds for years for the group storage. By sharing the commodities, I am reducing the individual slot requirement by the number of characters I have. Win.
The second option is the trade up. Inscription is a PERFECT example for this. You need to carry a solid dozen inks, milled from dozens of herbs in order to make glyphs. The trade up system already exists here where you trade 1 type of ink for another. This exact same model could be used for other tradeskills and not necessarily on a 1:1 basis. Maybe 5 gold bars gives me 1 truesilver.
A final solution is the elimination of grey items or, like RIFT, the addition of a “sell all crap” button to the game. The entire purpose of these items is to bloat bags and to be traded for cash. If you go the first route, simply change the RNG to give a cash bonus when a grey item should drop. If you go the latter, I know half my bags would empty in a second.
Increasing bag size is a temporary solution. The problem is that there is too much stuff that people collect in their travels.