One eye is bigger than the other. What is the cause of this? (Photo)

Myopia is usually caused due to the increased diameter of the eyeball. That is, the eye is usually bigger than normal. Since your left eye is more myopic, then it is larger than the right. Usually 3 diopters equals about a 1 mm difference.Looking at the photo, it does seem that the difference is more than that, however. This might be due to the way the photo was taken, as it seems like its a "selfie".An evaluation by an Oculoplastic surgeon, or a comprehensive Ophthalmologist would be a good place to start.

Considering that it has progressed over 2 years and looking at your picture your left eye looks more "bulging", I strongly recommend that you seek consultation with an eye specialist. 

From your photograph, it appears that your left eye if more prominent than the right. Any time that is the case, there are differences in the appearance of the lids that cover the eyeball. If this is a progressive or changing situation as you mention over the last two years, I recommend that you see an ophthalmologist (eye specialist) to determine the cause of the difference. Your vision is too important to risk. If a cause can be found that needs treatment, then take care of that first, and then any cosmetic concern can be addressed.John Standefer MD