I have wavy hair. When it's medium / short - ish (shoulder length or slightly above) I tend to find that it curls outwards / bounces up and becomes one big wave. A less defined shape. Whereas, when I grow it out a bit it falls into smaller waves that give it an overall curlier appearance. Of course, this might not be the case for you - it'll depend on your hair type. I have thick wavy fluffy hair which looks straighter at the top and curlier on the bottom layers. At the moment it's fairly long so it doesn't look particularly straight even on the top layer.
Often when I let it dry naturally, without using a hair dryer, it'll significantly curl. However, avoiding knots and frizz can be the difficult part. I have a detangling brush and I find this to be the most effective type for my hair. Lots of small bristles. Personally I tend to dislike wide tooth combs, I actually accidentally broke one in half once whilst trying to brush my hair on a particularly bad hair day.
Thankfully my hair isn't like that anymore - but I do have to keep on top of it.
Try to avoid over washing it - if you're washing it everyday, I'd try washing it every other day instead. One way to make it look a little thicker (or at least, this works for me) is to brush the hair, flip it over (like you're in The Ring ) - brush it - flip it back - then brush it back down. Of course, being aware of your surroundings whilst doing this is vital. I tend to do this when my bottom layer is knotted and needs detangling (this happens when I've been busy and neglected my hair - I haven't had to do this for a while) but it does have the side effect of making it look a bit thicker.
Disclaimer: Not a hair expert. YMMV.
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23. Possibly B.A.P.