Baymax

While she was in Antarctica in 2048, Clara Oswald felt like her layers of clothing made her look as big as Baymax. (COMIC: Blood and Ice)

Behind the scenes
Blood and Ice does not specify if Clara is referring to the Baymax of the Big Hero 6 comics by Marvel or the Baymax of the then-current animated film adaptation of Big Hero 6 released by Disney in 2014 (which was eventually released in the United Kingdom in early 2015).