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 1,500 years. (TV: Twice Upon a Time)
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)
Behind the Scenes
The TARDIS suffering a "size leak" in the Revived Series was to ensure that it was big enough for the actors playing the Doctor and their companions to fit inside for a brief time to create the illusion of it being "bigger on the inside". Pearl Mackie noted that the prop for the First Doctor's TARDIS was much more cramped than the one used by the Twelfth Doctor.
