The eye condition that made David Bowie's pupils appear different sizes was anisocoria.
Anisocoria means that the pupils are unequal in size. In Bowie's case, the striking appearance followed an injury during his youth: in 1962, at age 15, he was punched in the left eye by his friend George Underwood during an argument over a girl. The injury damaged the muscles that control the pupil, leaving the left pupil permanently dilated.
Because pupil size affects how much light enters the eye, the damaged eye could appear darker, creating the illusion that Bowie had two different-colored eyes. He did not have true heterochromia, which refers to a difference in the color of the irises themselves.
Cataracts affect the eye's lens, while ptosis is drooping of the upper eyelid. Neither accurately describes Bowie's famously mismatched-looking eyes. Anisocoria can have many causes, so a newly appearing difference in pupil size should be medically assessed.