Even in huge, say 125-200 gallon bowfront tanks, the glass is only 1/4" think? I would find that hard to believe! (But then again, I know nothing about bowfront tanks.) And what about tanks around 55-75 gallons?
So basically, one should not be able to see any distortion in a bowfront tank, right?
In some of the larger tanks it's probably 3/8" thick. When you temper glass it becomes stronger. Even when you see stores in a shopping mall with curved glass entrances that stand 12 feet tall, that glass is only 1/2" thick.
I would think that the larger tanks are a little thicker simply because of the pressure placed on the glass by the water. Then again, the upper and lower frames take some of the stress off of the glass itself.