How much can a rat drag/carry?

I’m in a campaign where it’s just me and my boyfriend, my boyfriend being the DM. He’s very literal with D&D rules and has argued that a rat can carry 15lbs. Rats have a strength score of 2. Carrying capacity is Strength Score times 15, halved if Tiny. I think this is completely ridiculous that a rat can carry 15 times it’s weight. Can someone help me out? Is he right? Am I right?