You're the fattest thing I've ever Done, it's a good thing you can't run

The story:

My mom and I were listening to it in the car and I ask, "Did she just say 'You're the fattest thing I've ever done, it's a good thing you can't run'?"
She restarted the song and when she listened, thought the same. I read the lyrics and sure enough, we laughed the rest of the way home when I realized just how wrong I was.