1) Pop bottles have better seals than disposable drinking water bottles. Drinking water bottles are lighter than pop bottles. You can put a pop bottle lid on a drinking water bottle and have sub-1 oz carrier.
2) Platypus weighs less than most options and works well. Even a 1 liter Platypus is under 1 ounce, so why should 1/2 liter be overkill?
……..2a. Platies seem to age faster when holding alcohol. This should not be a factor on treks shorter than 12 to 18 months.
…..2b. Never fill a Platypus much more than about 3/4 full and then press out the remaining air. This will keep it from popping if dropped. If the bag is close to full it can burst when dropped. Platypus used to include that caution with their bags.
3) The BPL pour-cap has to be trimmed a little (1/16 inch or a little more) to seal properly on a Platypus and some pop and water bottles.