nothing on the wowhead comments about either scaling with crit. the only thing i saw was someone thinking that big daddys work off spell hit - dunno if thats been confirmed or not. my guess is that both just have a base crit chance, unless someone posts back with through testing that shows it works off your crit level.
They are a fire spell and scale with your spell crit in addition to whatever debuff the person has on (ex: Critical Mass). Fire Specialization, Cataclysm (Destro. Warlock passive), Improved Soul Fire, Berserking, and whatever buff you have at the time also make them hit harder.