Left eye seems smaller than the right. Could this be due to weight gain? (photo)

General weight gain does not cause eyelid ptosis, but swelling can. The eyelids seem slightly swollen. See an oculoplastic specialist for evaluation.

This is not due to the weight gain.  Unfortunately it is difficult to see if you had this before the weight gain as it is hard to see both eyes in the pictures.  You should see an oculoplastic surgeon to evaluate your ptosis and see what would be the best option for repair.    

Congratulations on the birth if your baby! That's exciting to have a new addition to the household. Having just given birth, your body is going through a lot of physiologic changes. If your eyelids work fine and your have no vision changes then I would hold off on worrying about the differences in your eye shape. I see the difference and I also notice your eyebrow is much higher on the left. If this doesn't resolve in 3 months than see a bc plastic surgeon to evaluate your eye and eyelid and eyebrows. If your eyelid or vision or eyes are feeling off now then you should see an eye doctor soon. There are many reasons for eye shapes to change but for you I would let time pass and see how it comes back (which will with time I suspect). Congrats again!

ptosis after pregnancy is not unheard of.  In fact I have treated a number of patients who developed asymmetry in their eyelids after pregnancy.  Get a good evaluation with an oculoplastic surgeon before proceeding with anything.  With regards to her weight gain, 75 pounds is significant, so losing that weight will change the appearance of the soft tissues around her eyes but may not allow you to regain symmetry that you once had.  Also we have no clue how symmetric or asymmetric you were before because the pictures you posted have your eyes covered up.I'll attach some information you may find helpfulChase Lay, MDDouble board-certified facial plastic surgeon

I see your concerns but the before pregnancy photos do not allow both eyes to be seen! Best to see an eye doc to examine the orbital region.. 

First, you may want to consult an ophthalmologist to determine whether you have any issues with your eyes. If your eyes are fine, you should consult a Board Certified Plastic Surgeon.Best wishes