Usually a fatter ring gauge will have a milder profile. From what I understand, this is because the larger girth makes the leaf burn slower, hence cooler.
As for the taste profile changing, I could guess that different tobaccos react differently as the cigar heats up and the oils build in the cigar from the heat. I'd guess it's due to the cigar itself changing as it's smoked, not the tobacco leaf being different in parts of the cigar. If this is true, then Cuban leaf seems to be more susceptible to the effects of the burn, hence they are more "complex." Not sure about this one, but it seems to make sense.
EDIT: after thinking about this for a little bit, it occurs to me that this would also explain why certain blends or puros taste better at different humidities.
Again, I'm just surmising things here... I will defer to those who know more than I.
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