Go to the part where 'Big Big Person' Dad and all the mice have gone to view the cheese making premises. It says that the 6 mice were made bigger by Septimouse, and Oscar(the cat) stayed shrunk inside the basket. Then later on, when they are in a rush to get out, it says something like: 'Septimouse lined up all the mice AND OSCAR, and made them smaller again'.

Oscar was not made bigger in the first place, so how could he be shrunk again?