Correct me if I am wrong, but larger spur gears also give you smaller gaps between gear ratios. For a hugely exaggerated example consider the following.
You take a test in school. There are 10 questions (spur gear). If you wanted to score a 87 on the test (FDR) you only have the choice to get 8 right or 9 right. (pinions) to get as close to your goal as possible. If you have 100 questions on the test (spur) you can get 87 right (pinion) and hit your intended goal (FDR). Sorry for the convoluted explanation. I am a teacher.
