Well for starters this system of framing requires one to refer to framing lines which represent the true outside diameter of the timber.
The sizing that you see in that photo is required because the timber was overhewn it appears about 3\4". Now the upright timber that will stand in this spot is exactly the same length as all the rest of the vertical timbers that make up each bent of the structure. This sizing of the mud sill or plate at the contact point of each upright timber along the mud sill ensures that the upper ends of all the posts are right in line and will accept the upper main plate and make the roof line straight.
The upper main plate will also have to be sized at the point that the vertical timber contact it.
Depending on the hewing at times the sizing is hard to notice because of the accuracy of the hewing. I have seen barns where there was very little sizing at connection points but that was unusual with hewn timber.
I hope this answers your question if not just come back with more questions and we will continue to discuss this point until you are satisfied with the outcome.
