I don't pay that much attention to the weight on the labels as much as looking at the thread itself for this same reason.
I noticed the difference in the diameter/size of 40 wt cotton, poly, rayon, really varies throughout the different brands in my stash.
I haven't noticed it so much in the 50 wt because I bought so much and such large spools of the same brand that I haven't had to replace any of it yet LOL :D:D:D