There are a few things that can cause one eye to appear bigger than the other.
First, the eyes can actually be different sizes. Usually the eyes are not exactly the same size, but the differences are small enough that this could not be distinguished by the naked eye. There can rarely be very large differences in the sizes of the eye, such as one eye that is very much larger, but this is a condition that usually has been there since birth so that would not be part of this equation.
Second, the eyelids of one eye can be retracted and therefore you see more whites of the eyes one the retracted side than on the other side. This can give the appearance of that eye being bigger than the other. This is very common in people who have thyroid disease. In addition, in thyroid patients, the tissues behind the eye can get bigger and cause the eyeball to be pushed out of its socket, also giving rise to the appearance of a bigger eye. If you have symptoms consistent with thyroid dysfunction, then this should be an obvious place to start looking.
Lastly, if one eye's eyelids are droopier than another; if your eyelid on one side is starting to droop compared to what it used to look like, this can make this eye seem smaller and therefore the other eye seem larger in comparison.
Does this make sense and does this help address your concerns?
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My opinion is solely informative and does not constitute a formal medical opinion or recommendation. For a formal medical opinion and/or recommendation you must be examined by your doctor.