To me it sounds like mandibular tori. I have this same thing. It is extra bone growth. I’ve had it for as long as I can remember, and the dentist says it’s nothing to fear. The size can vary throughout your life and may sometimes cause discomfort. If it’s causing you pain, or you have any sores on them, I’d to go an oral sergeon or dentist. If they deem necessary, the growths can be removed. Hope that helps. It’s called mandibular tori.