Based on other reviews, I bought a L for my senior cocker spaniel, Martin. (Martin weighs 20-22 lbs. and measures approximately 16" from the top of his neck to the base of his tail.) The product size chart is inaccurate. To determine your pet's size, measure your pet AND read all of the other customer reviews because they're very helpful. If you're on the fence between two sizes, I recommend sizing up and then wash & dry to cause some shrinkage. Obviously, I can't guarantee that will work for your pet, but it was a successful solution for me and others.The Large-size fit is comfortable and only slightly too large for Martin, but it's not so big that the material significantly sags or gaps around his body. Before Martin wore it, I washed the jumpsuit in warm water and dried it on medium heat. The jumpsuit shrunk somewhat, which was my intention, and the overall fit was even better after the wash & dry. Also, the jumpsuit will fit Martin properly both when he's really furry and right after a haircut, which is also what I wanted.The jumpsuit knit is a soft fleece and isn't lined with any other material. It's a single layer of medium-thick fleece, meaning that the fleece isn't so thin that it feels cheap or paper-thin, but it's not so thick that you can't see light through it when you hold it up to a light or window. I don't mind the thickness because I intentionally purchased the jumpsuit for the purpose of Martin wearing it under his wool sweater on very cold & windy days -- the sweater only covers his frontside & front legs, and the jumpsuit will keep his backside & back legs warm. The fleece fabric also isn't so thick that it'll make Martin too hot when wearing both the sweater & jumpsuit, but the fleece will still keep him warm without the sweater. Also, the fabric doesn't have any stretch to it, but because the jumpsuit isn't body-hugging or too snug, Martin can still move & sleep comfortably without the fabric pinching or pulling.Overall, the quality is very good, and I'm hopeful that the jumpsuit (with or without his wool sweater) will keep Martin warm during our cold windy Nebraska winters.