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.
This is a Shapechanged Ancient Copper Dragon (as a rat) attempting to steal a large recipe book from a closed bag. This dragon is known to the character it is stealing from. So it's partly realism, seeing as how a rat couldn't realistically pull a large book out of a full bag. Even I have trouble getting a book out of a bag!
I think this is completely ridiculous that a rat can carry 15 times its weight. Can someone help me out? Is he right? Am I right?
Best Answer
According to the rules, yes, a rat can carry 15 pounds.
The Basic Rules section for Using Ability Scores describes how Strength and size affect carry capacity.
A typical rat is Tiny-sized and has 2 Strength, meaning that it can carry up to 15 pounds. It can also push, drag, or lift up to 30 pounds.
Is that realistic? No, but D&D isn't a physics simulator, and isn't very realistic in the first place. The 5e rules for carry capacity were likely designed to accommodate humanoid adventurers carrying armor and loot, hence the relatively high carry capacity.
You could discuss with your DM about using Encumbrance, an optional rule that imposes incremental encumbrance penalties on creatures who carry too much. Note that these restrictions would apply to all characters, in addition to rats. Details can also be found in the Basic Rules.