Most, if not all, glass thickness calculators aren't very accurate when it comes to building large aquariums because they only deal with hydrostatic pressure and linear deflection rates. The principals of the mechanical advantage, torque, levers, and momentum should apply to aquariums as well. The longer a material is of the similar thickness, the less force/momentum it will take to break it. Doesn't matter what it is. Example, it's easier to break a long wooden stick over your knee than it is a short stick. In an aquarium the longer the glass panels are, the more deflection or bowing there will be.....not just outwards, but inwards and outwards. It will bow outwards more and more in the mid-section of the horizontal span the further away it gets from any corner supports. Some of this stress in that mid span, will be reverted inwards (when bracing is used), which incidentally compacts the center bracing. This is why in longer tanks, sometimes you'll see the center bracing bowing upwards a bit. If that bracing is too weak, it can be crushed/cracked and fail. The longer that center bracing is, the weaker it is also, which makes it more prone to failure. The longer the tank is, the more stress there will be on the corner seams also since the longer length causes more torque on the joint. It's like using a longer shovel to scoop dirt/snow/ect. The increased stress in the joints isn't directly caused from the water pressure, but from the longer shovel (glass panel) pulling at it with that increase in overall deflection. So fishguy306 is correct also, to keep the safety ratio's in check, more bonding surface area is needed for the joint to be siliconed. So thicker glass should be used to hold the joint together and to decrease the deflection. Larger tanks are a different beast, where the increased dangers need to be realized and respected.
Another thing, most center braces put a big wrinkle in the safety factors. A safety factor is only as high as it's weakest link. For glass tanks, that weakest link is usually some flimsy plastic (or brittle glass) center bracing. Ideally, the top bracing should be equally as strong as the rest of the tank, but it's usually compromised since people want plenty of access to the tank. I'm guilty of that myself with the tanks I've built. Live and learn. A solid top with a few access holes is usually the best thing these days, just as long as you have thick enough glass or acrylic. You can bust a solid top as well, if it the windows bow excessively.
