Lithium is known to increase hunger in some. Bipolar itself is believed to have something to do with serotonin levels at least in part, though nothing is 100% certain. You're experiencing a normal side effect. Definitely a good suggestion from Weaver to try to satiate the craving with something at least wholesome when you choose to give into it.
I was ravenous in general on lithium, I just grazed all day long. Before I stopped taking it, I started eating only whole grain for carbs, avoiding pure starch, so I got the fiber, protein, minerals, and complex sugars all together. My weight stabilized by filling up on whole grain rice, quinoa, millet, or whatever. I went full psychotic still on Lithium, so not sure if that method would work long term or for you. Good luck.