It’s actually rocket science. A cigar-its length, diameter and cross-section (rectangular or round), and degree of filler compression (obstruction to flow) all influence the flow of air, and with it, the flow of the heat of combustion as the cigar burns. This flow of air and heat determine the combustion temperature of the “cherry”, which affects the mix of combustion products (what you smell and taste).
a. Tobacco combustion
At combustion temperatures that are neither too high to low, the taste and aroma of a cigar are usually considered to be at their most enjoyable. A smoking cigar is a dynamic system. Combustion produces water in the chemical oxidation process. This water is then drawn toward the head of the cigar, where it is partly absorbed by the filler, causing its rate of combustion to slow, and swelling the filler so as to reduce airflow.
Narrower cigars tend to burn at a higher temperature than fatter cigars. So the same blend will seem different when rolled into cigars of different ring gauges.
Square or box-pressed cigars have functional “corners” that channel cooler air around the center of the “cherry”. Thus the total airflow drawn through the cigar has a less direct effect on the combustion temperature of the “cherry”. With this alteration in the air flowing into the “cherry”, a square-pressed cigar will taste different from the very same cigar left in its natural, round cross-section. Sometimes, harsher filler blends seem smoother when the cigar has been square pressed.
b. Natural filtering
All cigars filter their smoke through the length of the remaining filler-moderating any flavors within the smoke. The longer the cigar, the greater the initial filtering effect. The thinner the cigar, the more these trapped chemicals are concentrated-sometimes to the point that a nearly black, oily, and bitter bead of condensate may appear at the cut head.
But with cigars of any size and shape, the flavor and aroma become stronger (perhaps harsher) as you smoke toward the head (the cigar butt-yes, the head is the butt). With longer and thinner cigars, you may reach this point with 2 or more inches of cigar remaining. With very fat cigars, you may burn your fingertips before tasting them. How much of an issue this becomes is highly dependent on the filler blend.
c. Smoke “volume”
A fatter cigar supports a larger cross-section of actively combusting tobacco, which results in more smoke, or denser smoke, per puff.
d. Unsmokable butt weight
Although a fatter cigar provides a somewhat lower combustion temperature, a greater volume of smoke per puff, and the opportunity to smoke the cigar to a shorter butt length, when compared to a thinner cigar, the weight of the unsmokable butt (a weight of tobacco that you have paid for, and committed to this particular cigar) is greater than with a thinner cigar.