I am on board with them weighing more... but lets be honest, you are making up temperatures now, we can just say Firebolt meets the requirements whatever number they may be... its a game of "can we take a real life example and complicate a fantasy world situation?"
the problem is players dont want to see other players do things the easy way and know that they had to struggle... or they feel like if the option is there to trivialize something, that they are wasting their time by not using it. We are still talking about a game, not a chore. Ultimately if they make the barrel tactic a chore, you wont see it very much at all except to make fun meme videos like this
I am the school that just because we have magic, we do not toss out physics just because, you know, I wizard did it.
OK, what about a flaming arrow. That one fire arrow can vaporize all the oil inside the barrel before it breaches the barrel and everything goes boom?