What dou think about it???
In The Dark Knight you actually see him building the gun-thing he used during his trip to China. Similarly in Batman: Arkham City it shows Batman's combining different gadgets together to make new ones. Though some of those things are still too big to all fit in his belt, I'm assuming the idea is that his gadgets are broken down into smaller pieces, making them easier to store in his belt, and he uses only the pieces he needs to build whatever gadget is required at that moment.
On another note, one could also argue that Batman only uses certain gadgets for certain situations. Having only his standard gear, such as his grapple gun, Batarangs, smoke pellets, etc. on him at all time. It should also be noted that, despite how it appears in medias such as the Arkham game series, Batman doesn't have a universal stash of weapons in his belt. He doesn't have an infinite number of Batarangs, but more likely has only a select number.