A typical, or average game, consist of the open, middle game and end game. 14 moves per each. There can be no hard rules on a miniature, but anything less that 10 moves must be a miniature. If I get beat in less that 20 moves I consider that a major thumping, but not necessarily a miniature.
The Definition of a Miniature

I don't think there really is a concrete definition like "25 moves or less," because there are many variables to a game.
Is it a miniature only if a mate is forced by move 25, or what if a player resigns before move 25 due to a forced mate threat coming after move 25?
I have a fairly loose definition for anything I personally call a miniature, and it also takes player strength into account:
2. Any mate on the board under 30.
So you don't think there is any concrete definiton like "25 moves or less", yet you propose your own concrete definition "mate in less than 30 moves". Double standard too much?
Also, your example "what happens if a player resigns before a checkmate that will come after move 25?" also applies to "what happens if a player resigns before a checkmate that will come after move 30?". So your logic and examples make no sense. You apply your examples to criticize the "25 or less moves", but then you follow a double standard and forget that those objections can also be held against the "mate in less than 30 moves".
The reality is that the vast majority of chess authors include as miniature any chess game of 25 or less moves. Only a very small minotiy claim that miniatures are up to 30 moves. And rightfully so, since a game of 28 or 29 moves is by no means a miniature sensu stricto.
A few references in chess literature (I could find many more):
"I have included games of up 25 moves as a miniature"
"Miniatures in chess are short games, generally 25 moves or less".
"Miniature (any game less than 25 moves)"

I just started becoming interested in searching for opening traps just recently, and just found out that aside from the "opening traps" term, there's another term which is "chess miniatures". As I look at the moves involve in these topics, how I define it will be "A short game, not reaching beyond 30 moves, which showcase a very unusual combinations. These combinations of course is a checkmate, or a huge advantage in material." At least that is how I can define it.
As you see, there are common themes and patterns involved for middlegame combinations. Regardless of what opening you use, there is still instance of common combinations that you can see from one opening to another. For combinations found outright in the opening stages, maybe around 7-15 moves, you can see that the start of the combination is very different to the point of that combination not viable for other openings.
So what I can see are these topics show the unique setup of traps and a few number of moves to finish the game. I define combination here as sequence of moves to force your opponent to fall for certain tactics.
I think my comment is a bit late here.

A Miniature is a brilliant, brutal and short chess game that makes you wish you could play chess like the winning player in that game.

Ironically the 13.5 move game of mine which started this thread ... had a defense that both players overlooked. Reminds me of the possibly famous Salo Flohr game wherein Black moves a Q or B on the queenside with some threat, and, as the book comments say, (Flohr) "thinks and thinks and ... resigns!"
I'd have to say there is necessarily a little subjectivity involved here. Is a 24 move game where someone resigned a pawn down in a boring position a miniature? Is a 26 move game with a scintillating attack culminating in a queen sac and mate not? Ultimately, I'd say, a miniature is just a pretty short game that's usually of the attacking/brilliancy genera. You decide what is and see who agrees or not.