It was written by Tom Defalco and he was very clear in the issue that Spider-Man got a lucky win and Firelord would typically be a tough fight for the Avengers.
The same Tom Defalco wrote the Silver Surfer busting a "planet" that Korvac created around him, Hyperion pushing Wonder Man through a small planet's core, and Thor containing a bomb that would've destroyed 1/5 of the universe. Except for Thor, none of the above struggled with these feats. I think the more likely explanation is that Defalco likes to give top tiers their space cheese.
In this particular case, I think he was deliberately showcasing Firelord's power to make it clear why Spider-Man beating him was a big deal. We take it for granted that everyone on forums like this knows who Firelord is, but back in the mid '80s, I'm sure there were alot of Spidey readers who knew little or nothing about the character before he appeared in that title (it was the first Firelord appearance I personally read). So it makes sense that DeFalco would want to establish what he was capable of early on.
The alternate explanation is that Firelord feared that colliding with a small asteroid would kill him. I don't think that fits with Defalco's body of work--he even had Iron Man crushing a mountain that Terrax hoisted.
But Iron Man flew into it, rather than it flying into him (meaning he had some control over the strength of the impact). And it still pretty much totalled his armour. Also, Firelord was stunned by inadvertently flying through an abandoned tenement, and ultimately KOed by a ten tonner, so it's not like his durability was obviously insanely high in this story.
One of the things I most strongly associate with DeFalco is his apparent love and respect for the Stan Lee era of Marvel Comics, and the extent to which that is reflected in his writing style. With that in mind, he'd almost certainly read the old Silver Surfer issue in which Spider-Man appeared. And in that issue, a couple of room-sized meteors appeared to present a very real danger to the Surfer. Even a fragment of one of them flying at him after he destroyed them was enough to knock him out cold. I'd be surprised if this story wasn't an influence on DeFalco's to some extent, considering the obvious parallels.
Also in regards to the art, it would be hard to show a planetoid-sized meteor flying toward a normal sized guy.
I agree, but Frenz could've made it look alot bigger than it did by a simple shift in perspective, putting Firelord in the foreground, with the meteor travelling toward the reader, rather than the other way around.
In any case, I allow some margin for error with these things. I could buy that this object was perhaps as large as a mountain. But I can't comfortably buy that it was anywhere near as big as the Earth or the Moon, say, because that requires too far a deviation from what actually appeared on the page.
It's not like it needs to be as large as the Earth or the Moon for the feat to be impressive and meaningful, regardless. In Spider-Man, we're talking about a character who was used to going up against the likes of Doctor Octopus and Hobgoblin. Compared to that, Firelord looks like a monster even if the rock he blew up was only the size of a house.
As regards Spidey vs. Firelord. While I initially thought the idea of it was nonsensical. Now that I have discovered Firelord was only in the 50-ton bracket (as opposed to a peer of Surfer himself) I'm actually okay with it.
That was his official ranking at the time, in a bio published just a couple of years before the story. One could debate whether it's an accurate ranking, but regardless, it's very possible that it was an influence on how the writer chose to portray the match-up.
In the Avengers issue which followed up on this story, the Black Knight was shown thinking to himself that it was incredible that Spider-Man could beat Firelord even if he was only half as powerful as the Surfer. Perhaps that shouldn't be taken literally, but it seemed to me like the writer (Roger Stern, in this case) was trying to allow for the possibility that Firelord and the Surfer were not truly on the same level.