Mark F wrote:I am curious as to why people use small collapsible bottles (500-600ml) assuming they are for water.
It mostly comes from the trail running side of things, where they are more comfortable sitting in the front chest strap pockets of a vest or vestpack, where they're easy to access and refill at aid stations or running creeks/water sources, and allow you to more closely monitor your fluid intake than a bladder hidden away. They also don't slosh nearly as much as a hard bottle as they empty. Being collapsible, you can stow a couple of extra empty bottles away in your pack, say for races that require you have the ability to carry a minimum 2L water yet the course maybe only dictates you'll actually need 2L of water on you to get through one section between aid stations or water sources... part of the Cradle Run in a dry summer for example.
I personally prefer them for hiking too, usually one 500mL plain water and one with a strong electrolyte mix. I've got a bunch of Salomon branded Hydrapak bottles in both the narrow and large-mouth versions, the latter being vastly superior for refilling and popping in electrolyte tabs/powders. They are quite expensive to purchase individually I'll admit (I usually get mine included with trail vests) but bear in mind they are covered by a lifetime warranty. I had a pair of the earlier design bottles start to leak at a seam point after a couple of years and these were replaced free of charge within a couple of weeks, no questions asked. The subsequent design is much better, devoid of sharp seams, and are still going strong after 3 years.