Jenifer, You are right. A majority of the weight gain would be from increased appetite as your physician has said already. I checked Lexapro's prescribing information off the fda's website and this is what I have found
In this pdf file they have Weight Changes: "pts treated with Lexapro in controlled trials did not differ from placebo-treated pts. with regards to clinically important change in body weight.
However it also goes on to say...
Metabolic & Nutritional Disorders: Frequent - Increased weight is listed. Then : Infrequent - Decreased weight
Weight gain was not listed on the Adverse Events Table which I thought was kind of interesting.