Why is Surtur so easily beaten by Thor compared to when Hela fights him?

Question

In the introduction of "Thor: Ragnarok" we are introduced to Thor locked up in chains and Surtur on a throne. Obviously Thor doesn't resist as he tries to get Surtur to spill the beans but eventually Thor defeats Surtur (too easily in my opinion) and returns to Asgard with his head/hat or something.

However near the conclusion of the film we see

Thor asks Loki to summon Surtur to destroy Asgard (the place not the people) and essentially destroy Hela

My question is, how did Surtur lose to Thor so easily whereas he defeated Hela a being physically stronger than Thor by himself and at the same time destroy Asgard with a few swings of his sword?

Did he lose to Thor intentionally because he knew it needed to happen for the prophecy to be fulfilled or was it just that the fire of Asgard made him stronger than he normally was?

Answer 1

If you listen to Surtur's speech at the beginning of the movie, he explains the issue here pretty well, you just have to remember it by the end.

Surtur, as we see him at the beginning, is only at a fraction of his power. Odin had beaten him and taken his source of power, the Eternal Flame, and hidden that away in Asgard. (Thor has actually believed that Odin killed Surtur, but he was merely banished to the world he's in when Thor finds him.)

So Thor is fighting Surtur at only a fraction of his true power, while Hela is fighting him after he's been reunited with his Eternal Flame, and is at his full power. Before Odin trapped him, he had been powerful enough to destroy all of Asgard by himself if he wanted, so once he's restored, Hela is no match for him.

Also, keep in mind that Hela, as Odin's true heir, was very closely tied to the health of Asgard itself for her strength. As the people left and Surtur destroyed it, presumably Hela grew weaker until they were both finally destroyed.

Answer 2

As noted elsewhere, Surtur is clearly more powerful at the film's conclusion (he's as big as a house mountain after all).

I would add the filmmakers deliberately do not show Hela's defeat, only that her efforts were insufficient to prevent the destruction of Asgard. I maintain that she very well could have defeated Surtur eventually, but in this case, not before Surtur razed Asgard.

As a throwaway point, I'll add that perhaps Thor went into his encounter knowing how to defeat Surtur, whereas Hela was clearly taken by surprise, may have been weakened from her recent battle with Thor, and plainly could not defeat Surtur in the few minutes it took him to level Asgard.