|Montage is protected by copyright|
This is a simple montage but I think that it is instructive. You can see that the Manx has been around a long time in the cat fancy and its shape has hardly altered. It is a bit more cobby (stocky) today than in 1903. That is because breeders like to push the boundaries a bit (or a lot).
In contrast you can see that the Siamese has gone through a transformation. In 1903 it was similar in shape to the Manx. Today it is super slender. This is due to selective breeding. The Manx today is considered a stocky cat while the Siamese is considered "oriental" in appearance (slender). The reason why the Siamese has become so skinny is because breeders at the CFA decided around 1950 that it should be more elegant. That is it. See Siamese cat history. See also old style Siamese.