Why does this happen if my tool is too big?

You appear to be using a custom StarterCharacter; the sword is likely pulling down the rig.

This isn't even really a matter of scripting (no i'm not saying go-on-and-moderate-and-downvote-and-reeeee); just enable CustomPhysicalProperties on the part(s) and set the densities/density to 0.01.

If that doesn't help, then my prescription: Collision Filtering...

--[[ https://wiki.roblox.com/index.php?title=Collision_Filtering
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                click here to learn more about
                     collision filtering
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