there is a limit on how much you can fit in it. Like all bins, it will fill up sooner or later, and then need emptying. If you right click on it, and select 'properties' it will give many options. select 'global' and this will show how much of you disk is set aside for the bin (as a %).
also on same page is the option 'do not move files to the recycle bin. remove files immeditly on delete' make sure that this check box is empty, not ticked.