If you can't read the chart correctly the 20in frame is a xl for a mountain bike. This is because they use fuckhuge top tubes and have suspension so you need longer legs to standover the bike, particularly in off roading situations.
On a road bike you don't need most of that, so a 20in road bike would be on the small side for you, but it would be fine.

>>2006926
>What am I misunderstanding?
You need to get of the internet if you've already started to make cope threads about your height. Sizes vary wildly based on manufacturer and geometry. For example seat tube length for a given rider's height on modern road bikes will be shorter than on vintage bikes because of how the top tube slopes down.

>>2006944
>anon asks a bike size question
>another anon spergs about heightcels
/fit/ is that way --->

>>2006926
5'7" is a cursed, in-between size. King of the manlets but midget of the kings. I'm about the same size with the curse of having long arms/legs with a short torso. You have to approach every bike individually because some smalls are too small, some mediums are too large, in every case, you'll need to adjust the stem lengths. It sucks being on the extreme ends of a size. Now for your 20.5in bike, it depends if it's a MTB or road bike whether it's oversized for you or not. If it's road bike geometry, then 20.5in downtube is close to optimal but if it's MTB geometry, then it's way oversized.

mtb geometry has been evolving since the 90s. wheelbase and more importantly reach have been increasing in modern bikes. so an older size large and a modern size small might have the same reach measurement. and an old bike probably has 26" wheels which will give you more standover clearance than if you were to ride a large 29" bike.
just because you can ride it doesn't mean it "fits perfectly" though.