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, implying it is slowly suffering from a size leak as it ages. (TV: Twice Upon a Time)