Sighthound coats are specially made for the tipical sighthound body.
For Whippets the back is formed in a bow, for the biger breeds (in the size table mentioned as Greyhound size) the back is flater. This is why the smalest Greyhound size 56 cm (22') is smaler than the biggest Whippet size (23').
If you are not sure about the correct size,
I will advice you, or also send a coat to try out.