Is a cat with a hernia dangerous?

yes it can be dangerous if it gets large and isn't treated, you mentioned it has increased in size I would get her to a vet. the skin and or muscle surrounding the hernia might be weakening. It may also not be a hernia, you will have peace of mind once you have it checked :) good luck. We had a rescue kitten once with a hernia from where he was attached to the umbilical cord, it was repaired when he neutered, the vet wasn't concerned at all and said the surgery to repair it is common :)