I've had some questions about this myself. I've always wondered if I've got the "best" Vitola for a certain cigar. I tend to be a larger ring person for some reason (Anything 48 on up).
I believe the primary reason that blends need to be adjusted for different vitolas is because of the wrapper/filler ratio. It's generally accepted that the wrapper contributes 40%-60% of a cigar's flavor, so adjustments need to be made to the filler to keep that same ratio for all vitolas in a specific line of cigars. Kuzi wrote a really good post about this, including math proofs.
I agree, it is the wrapper to filler ratio they are trying to preserve. That said, I prefer some sizes over others because they tend to taste better and hit the sweet spot faster. It also effects body in some sticks.
I'm with urbs, I really haven't tried all vitolas but I have noticed if I don't like a cigar, then try it again in a different size and still don't like it then I am almost sure a size isn't going to change it. I am sure that the blend stays the same, meaning the flavors and taste will be the same, just with different sizes you will get more or less of said taste. I know this is true with the camacho maduros, I really love the 6x60 over the robusto.
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