Here's the proof. http://imgur.com/a/JJxRd.
The irony here is the small and large bottles are on the same stand next to each other so the purchaser clearly has the choice to save 30c and get a larger bottle, although there could be the 'inconvenience tax' of carrying a larger bottle. Not that 600ml would last me that long on a hot day.
Down the next aisle you could get a pack of 24 smaller bottles for $6 (25c/bottle), which is the a typical price for a bulk pack, but not a purchase you would make to relieve your thirst.
Personally I reuse a bottle and fill from the tap for free. That's the OzBargain way
If I was thirsty in Coles I would:
the anger of your SO when you've done something dumb.
a diamond ring will get you less anger than a box of chocolates