system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are eyes generally bigger on bigger animals and smaller on smaller animals? Can't they be small for all animals?

Birds, for instance, have small eyes (relative to a human) and can see better than us.  So why do our eyes need to be so "big"?  I feel like if they were small like a bird's eyes, wouldn't they be less vulnerable?
gpt
I think a lot of it depends on what the animal uses its eyes for. Also, it might surprise you to know that eagle eyes and human eyes are about the same size. While owls have tube shaped eyes larger than humans. Also, elephants, the largest land mammal, has eyes only about 1/2 an inch bigger than the human eye. So, in a sense, your initial feeling is right. There is no inherent reason eyes have to get bigger or smaller with body size. 

Animals with especially large eyes, like the giant squid live at incredible depths where they need to be able to collect as much light as possible. Bigger eyes usually have more light-collecting cells and therefore can see better in the dark. Other differences in eye size can be typically be chalked up to how they're used or (for smaller animals especially) the eyes just fit what the body size will allow.