It really depends on the bunny. Some bunnies have a naturally thin frame, and some just look thinner with age. If she’s eating her food normally and running around fine, it’s probably nothing serious.

When Wick was slightly underweight (looked a bit similar to your rabbit’s current skin wrapping around bones), my vet gave me critical care, and he got as many syringes as he would eat, twice a day for a week. He plumped to healthy weight during that time and has sustained that since.

If something seems off, definitely see a vet. I know there are some things you can do to put on weight. I think some people give their rabbits oats…?

I use this as a guide, since it’s not based on weight but rather body shape/ distribution 

I was worried about my Peanut being underweight so I took him and Buttercup to the vets for a weigh in, then again a week later and again a fortnight later. (I wanted to be sure their weight was stabilised)

It actually turned out that Peanut is his ideal weight and has good muscle mass. Buttercup was a little chunky! But she lost a little and now she’s ideal too.

Basically you should be able to feel bones as though there is a layer of padding on them. If you’re unsure, could you get a vet to assess her body size/mass? Weigh ins at my vets are free

Panda and Fernando, especially the former, have a rather plump, round body shape, which, if my theory about them being Polish rabbits is correct, goes with the territory, since Polish buns naturally have compact, round bodies. (You can see pics in my thread in the Welcome section.) They’ve felt fine to me when I’ve petted them, but I’ll nonetheless take care they get no fattening stuff, as much as they may beg and plead for it.

Ruvik looks slender but, to my (inexpert!!!) eye, not dangerously so; she’s got a beautiful motley coat and a quite impressive tail, and seems to have not a care in the world at present, given that she’s doing a classic bunny-rug pose. If she’s eating/pooping normally, especially eating a lot of hay, and being active, and has a decent layer of padding over her bones as Sarah says, I wouldn’t worry overmuch.

Some rabbits are naturally slim. She looks like she might be slightly on the thinner side of the spectrum, but if you are concerned, as wick already mentioned, critical care can be given to help her gain weight.
But if she is active and happy I see no reason to worry too much about it!

Your bunny is gorgeous!

You would really need a vet to determine if she is the right weight for her size.

My Bindi is 10 and special needs so I weight him monthly and supplement his diet with Critical Care. Several TBSP per day.