Gene mutation makes mice as big as pigs?

It won't be so exaggerated. Gene mutation will produce some abnormal characters, but it will not be so outrageous, because in general, gene mutation is random, harmful and beneficial, but most of them are harmful. To be such a big individual, besides being able to grow up, it needs other functions to live freely, such as a strong heart, strong muscles and nervous system. And all the other related functions you can think of, and it is absolutely impossible for such multi-functions to mutate at the same time and evolve so harmoniously in a short time. Think about how long it has taken for humans to walk upright from monkeys to now, not to mention that most mutations are unfavorable because of such multi-functions and simultaneous co-evolution.

So its mutation will not make mice as big as pigs, at least not in the short term.