Well, that is only an internet regurgatation - I may not have a clear understanding. Close to the beginning of this thread, Dr. Diesel makes mention of the 1.9td having bad port design, and trouble with higher rpm's - rpms are a great way to make power. This and in some other posts makes me think this is so. However, sometimes the performance of the 1.6td is enhanced by parts from the 1.9td.
From my experience messing with my dad's 1.6td Samurai, it takes a bit more know-how to turn up the 1.6td vs the 1.9td in my rig.