[BugsJustFindMe]

Think of the motor as a barrel and you can decide whether you put the magnets covering the barrel walls or covering the top and bottom, and your goal is to have the most magnet surface. At short barrel lengths, you get more magnet surface if you cover the top and bottom instead of the walls.

The area of the top+bottom is (2 * pi * radius * radius)

The area of the wall is (2 * pi * radius * length)

For them to have equal magnet area, you need length to be equal to the radius, which is bad. That makes your motor really thick. In the axial design, you can make the motor thin and still have the same magnet area as before, so your motor now weighs a lot less.

[BugsJustFindMe]

If you want a visual of how significant this size difference is, this image shows the sizes of the two motor types in relation to each other at equal strength because the axial motor no longer needs any of the barrel length: https://imgur.com/qwe3tuH