I think what you're missing is the pitch of the gears. The TC4 came with a 48 pitch spur gear (pitch measures the angle of the side of the tooth). Because of the lower angle, the teeth are further spread apart. That means fewer teeth along the circumference of the gear. They tend to be stronger, but louder, and don't allow for as fine adjustment. There are also 64p gears. Since the angle of the teeth is steeper, more teeth fit on the same diameter gear compared to a 48p. So given to two examples you gave, the gears themselves are actually close to the same size, but the 64p gears (98/37) allow for more fine adjustment of the final drive ratio.