I got a thermal burn with blisters. why does the blister gets bigger and bigger and should i pop it?

Blister is damage: The blister is the full damage to the skin. Cold compresses will help. If the blister is tight or uncomfortable it can be opened but do not remove the skin. It will act as a biological dressing , like a skin graft and protect the underlying skin. When the skin is healed the skin will peel off just like a sunburn.

Normal process: You have at least a 2nd degree burn. This also leads to damage of superficial vessels in the 2nd layer of skin resulting in fluid (plasma) leaking out of the vessels forming a blister. The blister forms as the the damaged skin lifts up and gets bigger with time as more fluid collects lifting all the injured skin. Ok if blister pops and drains, but don't unroof as the upper layer is protective.