"The Dwarves were created by Aulë to be strong, resistant to fire and the evils of Morgoth."
-Tolkien Gateway, sourced from Of Aulë and Yavanna (The Silmarillion).
Relatively simple- Dwarves of all kinds should take only half as much damage, or even as little as one damage point, from fire and lava. (Or, for balance, only slightly reduced damage. Sigh.)