At track 0 the distance between two bits is much longer than at track 79, because track 0 is at the outer edge of the disk, but the disk spins with constant speed. There are some disk formats, that try to write all tracks with the same density, to store more data on a disk. The CBM 1541 can write at 4 different speeds, the Apple IIe uses 5 different bit clocks. The Apple Macintosh even changes the rotation speed of the disk drive to write 400 KB or 800 KB disks. There are other ways to improve the capacity of a disk.
Most computers write disks one sector a time.
There are other computers, like the Amiga, that write always the whole track. If you write the whole track, there don't need to be large gaps in the track. The Catweasel Extra format, uses a combination of these two methods and it uses a better coding method. There will be an even better format. Four sectors per track.