Not too narrow, not too wide.
We want an optimal opening for our bq bottle. It should be wide enough for most ice cube but not too wide to let drink spill while drinking from it. Most importantly, bq bottle is wide enough to clean by brushes too.
Not too thick, not too thin.
Thin stainless-steel wall is lighter in weight but too weak to withstand high vacuum pressure, resulting poor insulation performance. However, thick stainless-steel wall offers better insulation, but they are too heavy for everyday use. bq bottle picked the best optimal thickness which offer 24 hours of cold insulation and 12 hours of hot insulation but light weight to carry around.