First, pets are more often slim and tall rather than short and wide. So, the manufacturer of a short and wide flap is saying "get the height right and I'll give you plenty of width". The problem is that it's relatively difficult to measure how tall a flap needs to be for a given pet. (Here's a method, though, that works. Put the top of the flap at your dogs withers and the bottom no higher off the ground than 33% of this height. The difference between the two is how tall the flap needs to be at minimum for a normal, healthy dog).

A high aspect ratio flap is saying "get the width right and you'll have plenty of height". The width is easy to measure: Just open a door wide enough for the pet to walk through comfortably and measure that.

Especially for dogs around 20-60 lbs--which is a lot of dogs--there is a security issue illustrated by the flaps above.  (These are scale drawings of a very popular brand). A great many of these dog owners will look at the first flap on the left and say "width is ok but I need more height". Then they look at the taller flap that is the next available size up and see that the height is now ok but the additional width that they don't need has made the pet door large enough for a person to crawl through!!

The Endura Flap #8 is 8" wide by 15" tall. Not big enough for most people to climb through but large enough for many dogs that are too tall for the typical 'medium' size.