There are certain, highly speculative branches of physics that hint at ways to make an object bigger on the inside than it appears on the outside. For example, the entrance to the TARDIS could be an Einstein-Rosen bridge, ie a wormhole. This would allow the inside of the TARDIS to be located on another planet far away, and when the TARDIS travels all that moves is one end of the wormhole.
Alternatively, the inside of the TARDIS could extend into the fourth dimension, like cutting a hole in a piece of paper. Unfortunately, we don’t know of any way to cut a hole in our 3D space and wormholes theoretically require exotic matter with negative mass to stabilise them, which so far no one has discovered. Current mathematical models of the universe don’t prohibit such things existing, but that’s not the same as saying they are possible, or that we could safely travel through them.
Answered by Luis Villazon