There has been many hunter getting tells asking them how they got their pet so big. Some people have been saying it is the new pet biscuits from the trading card game but that only works for non-combat pets and it only doubles their size. This is an interesting little bug with 2.4. The pets owner does not see their pet grow but others do. So if you see someone asking you why your pet is so big or if you see a gigantic pet in the bank, now you know.