I get it.
I understand that restaurants need drink sales to make money. They represent a very high profit margin in an business where success is tricky. And I know that they can't afford to have people ordering water, then sneaking some coke into their cup. It's perfectly reasonable that water cups should be different from the other cups.
But do they have to be the size of a thimble?
I don't think it's just a "making sure you don't get soda" thing, either. No, at this point you're making a joke. I've got to either buy a real drink, or suffer your water shotglass of shame. I can almost hear them laughing each time I go for a refill*, drinking a full glass at the dispenser so I can get enough.
*Oh yeah? Well in addition to the calories I'm saving, I'm also getting exercize from walking so much, so joke's on . . . still me, somehow.