One eye suddenly smaller than the other. What could this be from? (Photo)

This could be from a number of different reasons. The most common which are a sinus cavity that is smaller on one side followed by possible thyroid hormone levels which can be treated medically in most cases. I recommend seeing your primary doctor to allow them to work this up prior to pursuing aesthetic options. Best of LuckDrC

Thank you for your question. It does seem like you have a slight droop in the right eyelid compared to the left.It would be best to see a plastic surgeon/oculoplastic surgeon to assess this and determine the reason for this and the appropriate treatment.Have you notcied a change in lifestyle recently - drinking more coffee/ late nights etc - tiredness and stress can lead to sudden changes too that may not require a surgical solution. There may also be the option of non-surgical treatments such as botox/ laser treatment that can help.The best thing to do is to see the surgoen and get assessed properlyI hope this is of helpBW

Please see an oculoplastic specialist for evaluation of new onset right upper eyelid ptosis (droopy upper eyelid). 

There is also a compensatory brow elevation.  The solution for this would be upper eye ptosis surgery.  I recommend getting yourself assessed by an oculoplastic surgeon.

Hi @Wasithea:
Droopy eyelids also known as eyelid ptosis result is caused by stretching of the eyelid muscle tendon AKA tarsoligamentous laxity. This can lead to the appearance of one eye looking smaller than the other and is easily fixed by tightening the tendon.
Ptosis repair surgery is often covered by insurance and if you have a droopy eyelid in addition to excess upper eyelid skin both of these need to be corrected at the same time with a Ptosis Repair & Simultaneous Blepharoplasty. 

Best of luck,
Dr. Karan Chopra
★★★★★
Minneapolis Plastic Surgeon
Face ⨀ Breast ⨀ Body Expert
Johns Hopkins Trained
Cosmetic Fellowship Trained

it looks as though the left eye is more prominent that the right - this may have been apparent in the old photo although it is more subtle. if your notice closely the height of the right eyelid looks similar but the left upper lid looks slightly too high, and appears to have a slight lateral flare, suggestive of thyroid eye disease. 
i would recommend seeing an oculoplastic surgeon for a complete eye, eyelid and orbital exam.
good luck

 From the limited photograph, it appears that there is ptosis present which is a weakness of the  Eyelid muscles that hold the upper eyelid open. Ptosis repairs usually performed by ocular plastic surgeon. Cosmetic eyelid surgery will be of no benefit.

You may be noticing eyelid ptosis.  This is sagging and a characteristic of aging.   If you have had a lifestyle change, that can contribute to the problem.  Consult with a board certified plastic for an in person evaluation.