It did show a difference of 3.6 degrees. However, most people who have used a variety of bags, including Western, have found that the WM bags are closest to their ratings. Anecdotally, my Antelope (5*F) keeps me warm below 0*F, my Summerlite keeps me warm a little under freezing, & my Mitylite right to 40. But anecdote is not science!
When it gets down to it (sorry, pun intended), if you have one bag with 70" girth and another with 59" girth... and both have the same amount of insulation in them, then the wider bag inherently can not be as warm as the narrower one. You're trying to use the same amount of insulation to fill much more area, which will lead to less loft.
I would be interested in seeing EN ratings for the MB bags, non stretched & stretched. I would be surprised if they were as close to their rating as the Summerlite.