I Just Started Taking Mirtazapine For Depression; 7.5 Mg Cut

I just started taking Mirtazapine for depression; 7.5 mg cut in half once at bedtime. I do not want to continue if I am going to gain weight. Is that likely on this small dose? Hopefully, I can get some relief at this small dose - or I will ask my doctor to change the medication. She prescribed it to me because I had major insomnia and no appetite. HELP!