Size leak

A "size leak" was, according to the Eleventh Doctor, when a dying TARDIS' "bigger on the inside" leaked to the outside due to a breakdown of the dimension dams. This happened to his TARDIS in an alternate timeline where he died during the Siege of Trenzalore. (TV: The Name of the Doctor)

When meeting his successor, the First Doctor commented that their TARDIS had increased in size. The Twelfth Doctor justified that it was to be expected, given it has been "tucking [its] tummy in" for over several centuries. (TV: Twice Upon a Time)

The size leak could happen two ways: the exterior dimension grows larger (TV: The Name of the Doctor), or the rooms in the TARDIS randomly pop out and redesign a building its currently in. (COMIC: Playing House)

A size leak would ultimately lead to a TARDIS "blooming like a flower" as it died, spilling out every single room from inside of it. If this happened on a planet, it would be catastrophic as all the rooms would spill out and potentially destroy everything. To prevent this, a Time Lord would often send the dying TARDIS to the heart of a star, so it could die harmlessly. (COMIC: ''Playing House)