K.,
there are other anti-depressants that are less likely to cause weight gain. your doctor should be versed in the options.
please DO NOT stop the lexapro cold-turkey. whatever dose you are at now you should begin to taper down 5mg at a time for 3 weeks or so (each 5mg). stopping altogether you might end up in the emergency department with all sorts of symptoms. meanwhile you can begin another SSRI or SNRI and slowly taper it up.
offhand, wellbutrin (buproprione)is also used for smoking cessation, and is especially helpful in curbing the weight gain that occurs then. it is non-habit forming, but also takes about 4-6 weeks to become effective. you can certainly taper it up while you taper the lexapro down, but please discuss with your doctor before doing anything!
best of luck to you.