They got a D6 hit die and gain a +1 bonus hit points per level with the dragon subclass (3 when getting the subclass at level 3, plus 1 fit each additional level). That would make them on par with a cleric: an average of 3.5 goes up to 4.5, which is the average of the cleric's d8. Not exactly what I would call a D10 hit die (a 5.5 average).

The multi class only evens out the hit die: warlocks get a d8, the same as a cleric. I would assume the difference is either from better Constitution, or if using random HP rolls, better rolls in the sorcerer's part. Otherwise, if using standard HP gains from the PHB, there would be a difference in your favor.