Size

 

I needed new undershirts so I bought two three-packs from two different brands.

One set is a perfect fit, the other is too large.  I measured from the neck seam to the shoulder seam and there is a 1 inch difference between the two brands.  Underarm seam to bottom there is a 2 inch difference.

Does "size" on packaging mean nothing at all?

How do you decide what to buy when things are in sealed packaging and you can't try them on?

Also:  These shirts say "100% cotton" but the fabric feels totally different than the "100% cotton" fabric on my older t-shirts.  What's going on?