I found the styles of Shaks to fit differently from each other also. I wear medium in the full zips, but found the half-zip to be quite a bit tighter in medium. I ended up with a large in the hooded shak (it is kind of big, but I like it much better than the medium fit). I didn't like the fit of the half zip and returned it.
I have 4 full zip Shaks, and 2 of them fit much tighter than the other 2. This is before being washed.