Big, squishy bumps on cats, especially in cats that go outside and are known to rough-house with the neighbour cats, are often subcutaneous (under the skin) abscesses.
Abscesses form when a cat is bitten by another cat. The cat’s sharp, pointy teeth pierce the skin, leaving nasty mouth bacteria there to fester into an infection. Pus collects under the skin and the pocket enlarges, leaving a hot, painful, squishy abscess.
Left to their own devices, these bumps will almost always rupture, making the cat feel better but creating a pretty big mess. Abscesses always require veterinary care for appropriate healing, including lancing and draining the pus, then providing pain relief and antibiotics. Sometimes we even have to place a drain under the skin to encourage the infection to vacate while the antibiotics work.
If your cat has a bump and it’s not an abscess, it could be any number of things, from a foreign body to a tumour. A sample of the mass should be taken with a needle and examined under the microscope to aid diagnosis and plan for treatment.