One eye being "larger" than another may either be a problem with eyelids or with the eyeball itself. If one eyelid is retracted more than another, or conversely more droopy than the other, the size of the eyes visible will naturally be different.
The position of the eye within the eye socket [orbit] can also affect the size of the eye. In thyroid eye disease, the eye can become more bulgy as the muscles/fat behind the eye may become enlarged. Complicating the issue further is a change in eyelid position [retraction] which can make the appearance even more obvious.
Rarely tumors behind the eye [usually benign] can push the eye forward making it seem larger. As mentioned previously an evaluation by an Oculoplastics/Orbital surgeon is necessary, sooner rather than later.