My Ribz is a medium. There are two shoulder straps and a strap across the back. All can be adjusted. The strap across the back would probably be what would be changed for waist size. It has an elastic section between the two adjusters and a piece of strap that parallels the elastic to keep it from being over stretched, which shows up as the loop in the back picture.
The incredibly hunky model is 5'-9" and 168 lb. with 34" waist. Maybe that should read "incredibly chunky model".
For this photo shoot, I loaded it with a dummy load of rags and put it on over a flannel shirt and down puffy and handed the camera to my grandson. As loaded, it fits just about right at its loosest setting. I could even get another layer or two under it and I can also adjust it quite a bit smaller if I could ever get my waistline to shrink.
Both the medium and large will fit 34" to 38" waist and can be tightened smaller than that.
My original RIBZ was of a different design and was larger. This is the 8L model and works well to augment a pack. It's a little small for me as a daypack when I'm hiking with the grandkiddos because I try to carry more emergency gear when I have them with me. If I was planning on using it more as a daypack, I'd probably opt for the larger 11L size.
I've used the RIBZ under a shell to add more capacity to my carry on luggage when flying so as to avoid ridiculous checked baggage fees when possible. I did attract some undue attention from TSA a couple years ago at Boston Logan. I met the scary demographic of "old white guy with camping gear" and acquired my own personal humorless attendant through the entire line who wanted me to keep my hands in sight at all times. My hands and all my gear got swabbed and everything went through the X-ray machine twice. And to think I bought the green one because I thought an older black one I had looked too tactical.