As I'm getting older, it seems one of my eyelids is either getting bigger or the other is getting smaller (Photo)

One of your eyelids is drooping relative to the other.  This is referred to as Ptosis.  You are also raising your eyebrow to compensate.  The eyelid can be corrected.  I do not see a need for brow surgery.  See a specialist for a consultation.  

Thanks for your photo! As you age it is not uncommon to see an asymmetry become more noticeable especially on the face. Just from the photo I can see a few things going on. It appears that you may have a slightly lazy eye or strabismus on the left (that could also be the camera angle). I can see the fold in your right eye that is not present on the left. There are couple of options for reducing the appearance of the asymmetry but nothing that you do will make your eyes exactly the same. Faces are not perfectly symmetrical. You could have a brow lift using an endoscopic technique or you could have a blepharoplasty on the right eye to reduce the crease in the lid. Since your eyebrows appear to be anatomically in the correct place, I would probably recommend the unilateral blepharoplasty.  Please see your local board certified plastic surgeon for an opinion. A physical exam may yield something else. 
Good luck!