the problem is they had to give the goblins 3 times the hitpoints as a balancing mechanic for their unnecessary elevation system
So goblins should have 7 hit points right?
Say a cleric starts with 10 or 11 hp. 10 being 8+con mod of 2. Then lets give them 2 levels at a 5 (which is avg given according the PH) and add 2 for con mod. So thats 10+7+7 = 24 HP. Wait havent we seen lvl 3 goblin clerics at 24 hp?
Take a note at the health pool of the goblins, especially in the ruined shrine courtyard where you found volo. They are level 1 humanoids who happen to have specific classes and can do what those classes present for the most part. So wouldnt it make sense that their Health increase while they level?