Dimensional control
From Tardis Wiki, the free Doctor Who reference
The dimensional control was the control system in the TARDIS which controlled the link between the dimension the interior of the TARDIS was in and the dimension the exterior was in. It therefore meant that the TARDIS could have a larger interior than exterior. If the dimensional controls were damaged the interior would become the size of the exterior because it would not be in its own dimension. (TV: The Time Meddler)
