Bonus dice are not simulating anything specific; they are for anything that jazzes up the players (including the GM), whether by being cool, advancing the plot, being especially clever, etc. Not for "I have a bigger gun" or any mechanical or Simulationist concern.

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The in-game imaginative "weight" of the thing might be the in-play expression of the jazz-up. If a character pulls a bigger weapon than the other guy, then a +1 die to start off with is nifty. From a Sim standpoint, the machete gets an extra die for its bigger heft and longer edge in the "reality" of the game-world. From a Narr standpoint, it gets an extra die because our guts churn to look at it, especially when it's first revealed.

It is perfectly OK for the Narr approach to rely heavily on the in-game-world plausibility factor. In that sense, the effectiveness of the imaginary/fictional weapon if it were real is definitely a good justification for the guts-churning. That's why weapons have different damage effects in the damage charts in the first place. But this is a matter of relying heavily on, ***not*** being defined by. [1]