Are my eyes big and prominent because of weight loss years ago? Can it be fixed by gaining weight? (photo)

Prominent eyes can occur for genetic/familial causes as well as thyroid problems and rarely tumors.  Weight changes do not effect how much they bulge or appear proptotic.  It appears you have blepharoptosis of the eyelids (the lid margin hangs low), but without an in-person exam its not possible to give any further explanation for your concerns.  Best regards. 

You are correct to note that eyes do change shape (in many different ways) with aging.From your photos, it appears that the prominence of your eyes is a result of the size and shape of your eye socket and the size and shape of the skull bones around the eye socket.There are numerous procedures that can be used to make the eyes appear less prominent, including building up the edges of the eye socket and cheek bone with filler or fat grafting

Weight fluctuations typically do not affect the orbital appearance to any great extent. Myopia (near-sightedness), thyroid issues, and a few other maladies can make the eyes appear prominent. If you are a healthy weight for your height, then I would not recommend gaining weight for the purpose of changing the eye appearance. 
You appear to have substantial ptosis on both sides, and this could be repaired with levator surgery. Seek consultation with an Oculoplastic surgeon in your area for evaluation and treatment. Best wishes.