Does the drug Mirtazapine (Remeron) cause a decrease in metabolism as well as an increase in appetite?

I’m just curious – I’ve been on Mirtazapine 15mg for about 6 weeks now. So far I’ve gained 15 lbs (which is fantastic for me since I was severely underweight). I know that weight gain is a common side effect of this medication. But I’d like to know what causes it. Is it just the increase in appetite? Or does it also slow your metabolic rate? The reason I’m asking is because lately I haven’t been eating all that much, but still seeing a steady weight gain so I’m wondering if there’s been a change in my metabolism.