"the reason you have 200lb draw weights for xbows, compared to 60-70 lb draw weights for longer axel to axel compound bows."
This is easy to exlpain....The power stroke on a horizontal bow can be 12 to 16", where as on a vertical bow it can be twice that(approx) With a longer power stroke, the arrow has more distance to achieve a given speed. With the shorter power stroke, more power is required of the limbs to achive the same arrow speed of a vertical bow with 1/2 the power.
As for the shorter arrows being unstable....(from a bench rest as I shake when standing) using the high-end excalibur Exomax(350fps) I can make a 4 to 5 inch group at 100 yds, on targets. Disclaimer:I had a 16 foot plus arrow drop at this distance...do not attempt to hunt at these distances.
The physics of both bows are very similar.....limb strength, power stroke, arrow weight...etc.
I am glad that you found the information you were after. I was just not sure why you thought the physics of horizontal bow were inferior, and I was looking for the resoning. Something for me to ponder on,...I guess.

Todd