I am not allowed to have a magic cube that can expand to any size I want. Especially if I get swallowed by a dire shark in the middle of the ocean. Especially if I try to use the cube to escape said dire shark. Especially when the DM says "what size" and I say 100' cubed. That is especially large enough to destroy the ship the other PCs and campaign Macguffins are on. Especially if in my genius I decide to then shrink the cube to help .. thereby displace all the water it was floating in, creating a vortex that killed the unkillable Macguffins and nigh drowning the entire party.
All following campaigns have a cube joke somewhere in them.