I am 5' 4" weigh 149.5 lbs and I have an athletic build. I am fairly toned but have a little back bulge. I have a short waist. The Medium fit snugly on the first set of hook and eye closures. That way you can judge by the measurements. I used a tape measure.I had a hard time getting into this the first time. It is rubber/thick latex does not give much. I have had it for over two months and this is truly a waist trainer. It will not flatten your tummy. I purchased the medium and it fit very tightly and I believe that is the way it is intended to do. It really pulls in your waist and if you have a fuller bust the girls get pushed up ALOT so you may not look as svelte as the model does.The model depicted appears to be a smaller frame, with a small waist already and has a long torso so it fits her better and her bust isn't spilling because more than likely she is not wearing a garment to squeeze as much as most purchasers.In real life I ordered the vest instead of just the waist cincher because I was afraid of bulging back fat. I am not extremely big or over weight but I lack a lot of muscle tone along the middle of my back along the sides and KNOW that it will create an unwanted bulge underneath clothing.I wear a padded push up bra and it make the bra gap because it changed my underbust measurements so be prepared for than. When I wore it to the office under my dresses it did give me an hour glass appearance but it does not flatten your tummy. Your tummy will smooth with a rounded appearance.Wearing the vest made me sit taller, my waist noticeably smaller and my back fat was smooth not spilling. I had some irritation underneath my right arm (I think from using the mouse), it buckled a little along my rib cage because it is a little long for me (I sometimes get pain underneath my breastbone and wearing it while I was driving was uncomfortable at the time), it rubbed a little on the front of my hip and those are the predominant reasons why I stop wearing it. I wore it consistently everyday except weekends for over a month to work and also worked out in it at the gym.As I mentioned before tt was hard to get on the first couple of times until I got the hang of it. For me I bend at the waist and hold each end of it to get some muscle and as I come up I hold my self in and pull it out just enough to get one hook it into the other side some where near the center. Then I match them up at the bottom and the top and reinsert the mismatched hooks. Most times I never hook the right eye so I color coded mine with nail polish so I now where each hook matches perfectly the first time. This saves time and energyI haven't worn it in a while s only because I had been training at the gym after work and it was causing irritation around my underarm.Since my purchase I have managed to lose 5lbs so I hope it will feel a little better when I put it on again and I wholeheartedly believe that if you wear it on a consistent basis you will get a smaller waist line and I think it would work just as well if you ordered a size up if you are not in the process of losing weight like I am.This company also makes undergarments and I am more interested in those for creating a smooth silhouette and for the time being I am not giving up on this vest. But in the future I will order the waist trainer along because of the irritation I experienced under my arm.