My rabbit is underweight... :(?

If they are in the same cage, you could not put your underweight rabbit's food in there. That way, the overweight rabbit wouldn't be able to eat the other's food. Remove your underweight rabbit and feed her separately. I would recommend feeding her black oil sunflower seeds, which you can get at any store that sells bird seed. That's what it's sold as. Only give her a little bit, because a little goes a LONG way! Also, even if she is grown-up, feed her alfalfa hay. It should usually just be given to babies and pregnant mothers, but it will help her gain weight.

Hope it helps!

What are their ages and what are you feeding them? They should have constant access to hay. Hay should make up the bulk of their diet. If there is constant hay in the cage (refreshed twice per day) then the smaller rabbit will get plenty to eat.

Pellets should be limited (if buns are 6 months or over). Rabbits can over eat pellets and that will cause them to become obese. Hay will not cause obesity. It sounds like the larger one may be eating too many pellets and the smaller one isn't getting enough hay to compensate.

Take a look here for more on diet:

http://rabbitsindoors.weebly.com/feeding.html

You'll need to separate them if the big one is preventing the small one to eat (changing food isn't going to help that and will end up making the big rabbit fat if they have access to the same food).

If you can restrict the big one off from an area of the cage/hutch/wherever they live with an entrance only the small one fits through, that can work too.

I would check the small one over for missing fur or bite marks, too. It's possible if the big one is preventing it from eating, she might be attacking the smaller one, too.

You might have to put the underwieght one in a different cage at least during feeding time and free feed it. I had to do that with my new bunny. Also if the other is over weight you will have to limit it's food till it is at a healthy weight or it may get sick from obesity.

feed them more.