The Yawl's mizzen is substantially smaller as well. My Pearson has the same sail area as the sloop, without the mizzen. A Ketch has a smaller main than the equivalent sloop, but has a large mizzen to make up the difference.A yawl's mizzen mast is aft of the rudder post.
A Ketch's mizzen mast is forward of the rudder post.
This is the main difference between the two. The sailing techniques are a tad different also..