The weight is just the weight and does not imply quality, well to some people it might but basically its just the weight. I would expect a heavier weight shirt to wear longer and most work shirts sold are 6.1 oz.
Soil, water, pesticides, sun, handling, etc. etc.so what determines quality?
Usually the shirts are cut generously to allow for some shrinkage after washing. For example, Anvils shrink much more than Gildans, but the Anvils are also larger pre-wash to allow for the shrinkage.What really determines quality is how well a shirt holds up under repeated washings. I have used Hanes Beefy tees, only to have my customer call & say the shirts shrank up about 2" only after 2 washings.