Board Thread:Questions and Answers/@comment-3563147-20131203210416/@comment-5230978-20131203214953

Index took Sphynx with her on August 8 (Deep Blood arc). Touma, after being away home for almost three weeks, noticed that the cat was bigger than what he remebered (Homecoming arc). Sphynx clearly grew and is safe to assume that even Misaka Imouto's cat did the same.