Several factors are at play here and they all combine together to show us how Surtur would have defeated Hela.
Firstly, you are correct in assuming the fights between Thor and Surtur and Hela and Surtur aren't comparable. Thor was fighting a weakened and injured Surtur whereas Hela was fighting Surtur reunited with the Eternal Flame and at planet killing size/power.
We then have the fact that Hela herself was probably not at full power yet as the longer she was on Asgard she was still drawing her power from it.
Heimdall: She draws her power from Asgard and grows stronger everyday.
Thor: Ragnarok
I think it's also worth noting that just because she is extremely powerful does not mean she is invincible or undefeatable. Hela has her limits to and that happens to be a full sized Surtur, the planet killing kind. Her reaction to Surtur breaking through the palace also gives some hint that she's not entirely confident herself that she will be able to defeat him.
Hela: No...NO!
Thor: Ragnarok
And of course it's worth mentioning that Hela has been weakened. The fights for the past few hours/days may not have taken too much out of her but just before she fights Surtur Valkyrie shoves Dragonfang through her back and Thor unleashes a massive amount of lightning into her sending her into the water below.
Also whether you believe Asgard is the people or the place, by this point in time both have been sufficiently weakened. And so Hela's power has presumably been diminished somewhat.
It's worth noting that Hela took a direct hit from Surtur, if she hadn't been stupid and moved around a bit she might have dodged the Twilight Sword and not been killed. Instead she got stabbed directly with it in her last ditch attempt to defeat Surtur attacking head on. The Twilight Sword goes directly through her on its way to Asgard's core.
I also think it's worth noting that whilst we know it was Odin who defeated Surtur, we don't know any of the details surrounding it per my knowledge. Presumably Odin would have had Asgard's army with him and maybe even Hela at that point to help him out. Hela, on the other hand was fighting on her own against him.
Thor: Surtur. Son of a bitch...you're still alive! I thought my father killed you, like, half a million years ago.
Thor: Ragnarok
Lastly, on the note of prophecies, the Ragnarok prophecy had already begun and there was no stopping it. Whilst you say it could be delayed I don't think that's how it works. Both conversations about the prophecy state it is already underway so there's probably not much room for delaying in the middle.
Surtur: But Ragnarok has already begun. You cannot stop it. I am Asgard's doom, and so are you. All will suffer, all will burn.
[...]
Odin: I failed you. It is upon us...Ragnarok.
Thor: No, I've stopped Ragnarok. I put an end to Surtur.
Odin: No. It has already begun. She's coming. My life was all that held her back, but my time has come. I cannot keep her away any longer.
Thor: Ragnarok
