Marshall Allen wrote about the size of eye drops:

• Each medicated eyedrop typically delivers more fluid than the eye can hold. The extra medicine leaks away uselessly out of the eye.
• For patients who rely on the eyedrops to preserve their vision, this is a waste of medicine and a waste of money.
• Studies have shown that smaller drops not only effectively deliver the right dose of medication, but also reduce some of the discomfort eyedrops can cause.
• But the wasted medicine is a revenue generating boon for manufacturers – who earn $3.4 billion from eyedrops a year.
• There are regulatory hurdles as well. Any company that wanted to introduce smaller sized drops would have to obtain FDA approval and invest in extensive studies.

Read more on NPR.