Human eyeballs are almost the same size in every adult, and they stay remarkably constant across a person's lifetime — unlike ears and noses, which keep growing as people age. At birth, an eye is already about 70–75% of its adult size (roughly 16–17 mm in diameter for a newborn), and it reaches full adult size, around 24 mm in diameter, by about age two to three. After that, the eyeball's growth essentially stops, so a person's eyes look proportionally larger on an infant's or young child's face simply because the rest of the face and skull are still small in comparison.
Among adults, eyeball size varies only slightly from person to person, generally by a millimeter or two, and differences between the two eyes of the same individual are usually negligible. That said, "eye size" as most people mean it — the visible, apparent size of the eye on the face — is not fixed the same way. This appearance is shaped by eyelid shape and position, the amount of visible eyeball versus eyelid, orbital bone structure, fat distribution around the eyes, ethnicity, genetics, and even eyelid drooping (ptosis) that can develop with age. So while the actual anatomical eyeball is nearly constant in size, eyes can look bigger or smaller due to these surrounding features, swelling, fatigue, or aging skin.
Certain medical conditions can also cause true or apparent size differences: anisocoria (unequal pupil size) makes eyes look different even though the eyeballs themselves are the same size, while rare conditions like microphthalmia (abnormally small eye) or buphthalmos (enlarged eye, often from glaucoma) involve genuine differences in eyeball size, usually present from birth or developing in specific eye diseases. Significant, sudden asymmetry in eye size or noticeable bulging in one eye should be evaluated by a doctor, since it can signal an underlying issue.