There may have been an idea behind it like flavor concentration, but I think it really just comes down to product differentiation and producer preference.
Sizes with tapered feet (perfecto, salamone, etc) make the cigars easier to light.
In a lot of cases, the shape definitely affects the flavor. Try a fuente chateau and then try the same in a double chateau. Same blends, two very different flavor profiles. Or, try a robusto and then try a lancero of the same cigar. Different shapes/sizes affect many of the cigar's characteristics. For example, with a smaller ring gauge cigar, you'll get more of the wrapper and binder in the smoke where a larger ring gauge will yield more of the filler. A torpedo is going to direct the smoke to a different part of the tongue and trap the oils produced from the burn differently in the tip than something with a rounded cap. Finally, a box press will sit differently in the mouth giving more flavor on the lips than a round cigar. I'm sure the more educated can give better examples, but these are things I've noticed so far.
For those who are curious, try a sampler where you get different shapes and sizes of the same blend for one of your favorite cigars. I bet you'll be able to notice a difference. I've found that I like robustos and torpedos the best for most cigars, though I'm looking forward to trying the DPG lancero I have sitting in my humi right now.
P.S. As a side note, most blenders will have a favorite vitola, which they'll reserve the best tobacco/rollers for. Examples include: CAO box press, Fuente Anejo sharks, and lancero or small ring gauge cubans. It's just all depends.