First you mentioned shrinking draw length in the title, then draw weight in the first post, then draw length again. What are we really talking about here? Both maybe.
Even if you draw it precisely the same distance to your anchor as you always do, your personal draw LENGTH will 'appear' to shrink if you check it to the back of a bow with a thinner handle.
If instead, your are checking draw WEIGHT on various bows, and draw an arrow marked at 'your draw length' to the back of the bow... when you check a bow that is thinner in the handle section, you will be drawing if farther and getting a reading higher than what you will really be pulling when you shoot it.
These incorrect and inconsistent lengths and weights are some the reasons for the standard.