Tell me what happens when the first one succeeds at 'belch forth flame', and then tell me what happens for the second one...
This doesn't quite make sense, because in Dungeon World the GM doesn't declare actions for their characters/creatures that might then succeed or fail. It's the players who declare actions, and the GM makes moves as appropriate in accordance with the general rules of the game.
I don't know DW well enough to know what is implied by "Belch Forth Flame." Is that a Hard GM move? Is it something I am supposed to just make up on the spot? Are there rules for how much damage that sort of thing is supposed to do?
"Belch Forth Flame" is a description of what, in the fiction, a chimera does (or tends to do). And it also serves as advice, in the rules of the game, to the GM as to how to colour their moves. If the GM opts to
deal damage as a move, then "The amount of damage is decided by the source" (p 167). In the case of a monster, the
damage entry in its stat block "is a measure of how much pain the monster can inflict at once. Just like player damage it’s a die to roll, maybe with some modifiers. A monster deals its damage to another monster or a player whenever it causes them physical harm" (p 222).
There are many ways the GM could use the presence of a chimera in the fiction to make moves in play. For instance, suppose that the GM has described the chimera standing in a cavern with a great statute behind it. The players say "We advance towards the statute." That's not a player-side move, so triggers no roll; it invites the GM to make a soft move in response. Maybe the GM says "As you advance towards it, the chimera belches forth flame. What do you do?" This is
putting them in a spot.
The players are free to respond as they wish. One might declare "I dive out of the way of the flame!" That would be Defying Danger based on DEX ("When you act despite an imminent threat": p 62). If the player misses the roll (6 or less) the GM can make a hard move in response - perhaps
dealing damage: "The flame sears you as you tumble across the floor. Roll d10+1 to see how much damage you take" (in DW, the player always rolls the damage their PC suffers).
Another might declare "Heedless of the flame, I charge the chimera and chop at it with my halberd!" That is handing the GM a golden opportunity, and so the GM might make their hard move - "Roll damage to see how much the flame burns you" - before then calling on the player to resolve Hack and Slash ("When you attack an enemy in melee": p 58).
My point is that it does not prescribe what happens at all, it leaves it wide open for the GM, and that is what makes it narrative.
I think you are exaggerating this difference between DW and 5e D&D. The rules of 5e D&D don't tell us what happens when the PCs encounter a chimera in a chamber standing between them and a great statue. Maybe it breathes fire on them. Maybe it ignores them. Maybe it flees from them. The GM has to make those decisions, based on their sense of the fiction and any relevant mechanical processes (eg maybe a player declares that their PC casts Cause Fear on the chimera).
The difference isn't about "prescription" vs "wide open", it's about the process of play used to prompt and resolve declared actions.
In my view, what makes Dungeon World "lighter" in its rules, compared to 5e D&D, is not the length of the rulebook (my copy, from which I've quoted above, is a 410 page PDF) but the fact that it has fewer sub-systems or specialised processes. The version of D&D that comes closes to that is 4e D&D, but it still has more distinct processes than DW (especially its very different processes for resolving combat vs non-combat).