what causes big eyes in people
Other reasons are simple correlation or genetic reasons. Bigger people in general have bigger eyes and bigger eyes can run in certain families. Finally some people can have the illusion of having big eyes due to proptosis - this often is caused by thyroid disease but there are many other causes. Also it is important to be specific even though they are again closely correlated do you mean big eyes or big corneas.
There are various factors involved in the eye appearance. The most common cause of large eye appearance is actually wide open eyelids. Another cause is bulgy (protruded) eyes. Rarely it is due to actual large eyeballs.