My cat will not stop peeing over the box

Before you ask, my cat has a perfect bill of health, besides being overweight. For the past year, she has been continuously peeing over the backside of the box. At first, I chalked it up to be a territory thing with my other cat. Fast forward to now, she is the only cat in the house and she is still doing it. And I’m not talking about little drops, she will fully pee standing up, right over the back of the box. I have gotten her the biggest/tallest box I can find and yet it’s still happening??? I literally don’t know what to do, she has ruined 2 carpets and the paint on the wall. Even the ones with tops on them, the pee will seep through the crack between the top and bottom and still get on my floor. Please help!