I'm sure this isn't going to be a popular game fix, but rental cubes now have a limited amount of space to store your gear. Cubes are a cheap place to rent and are not the largest of places to live, so storing vast amounts of gear isn't exactly what they're designed for. Once the amount has been reached you will not be able to drop anything else in the cube. They still have a decent amount of storage for most players. Anyone with tons of gear should probably search out a larger place to live.