@Retreater - it's not that
bites are soft moves and
wing-generated fire tornados are hard moves. (Or any other similar framework.)
Soft and hard moves are categories of
GM narration. The monster (and its damage stats and tags) are tools to help give shape and content to the narration.
So if you are making a soft move (say,
show signs of an approaching threat or
reveal an unwelcome truth) that might reveal something about a monster that is in the scene - eg that is has teeth as long as knives; or that it wings can whip up fires that would destroy a peasant's hovel in minutes. Or the example that @Ovinmancer gave upthread - you see the maw of the dragon yawn open and inside it mouth and throat the fire is gathering, about to spew forth in a terrible blast.
What do the players have their PCs do in response to your question "So what do you do?" This will determine what sort of move you make next and how hard or soft it is. For instance, if a player narrates in response to your description of the dragon about to breathe fire "I ready my shield to protect me from the dragon's fire" then that sounds to me like CON Defy Danger. On a 10+ it works. On a 7 to 9 you as GM have to offer a hard choice - say
either fall back, or lose your shield. If the roll is 6 or less, it seems to me like the shield is burned away and probably damage from the fire is also fair game.
Rather than
actions per round or
uses per day, the relevant consideration is fairness, fun, and the logic of the fiction. So suppose the PC survives the fiery breath (with or without shield). To me it seems like it would be "cheating" - cheap on the GM's part - just to narrate more fiery breath. That would be to negate by fiat the successful defying of danger. If as GM you still want to focus on the breath, you could narrate something softer like "The dragon finishes breathing but you're still standing there. It pierces you with its gaze while inhaling deeply. What do you do?" Now the player can declare that s/he rushes in to Hack and Slash, or that she runs over to the cliff to unchain the prisoner, or whatever else s/he wants to do. Different response there may require different rolls (eg Bend Bars for the fighter to tear the prisoner's chains out of the rock) and may make it fair or unfair to deliver another breath of fire as a hard move.
TL;DR: the hardness or softness of a move is about pacing, dynamics of play, being fair while maintaining the pressure; the stats of a monster establish the parameters of what the fiction allows for when the GM narrates it.