Are there any beverage containers that can get smaller as they hold less fluid?

Those disposable baby bottle liners are like that: a flexible plastic insert inside a holder. This sort of thing. As the baby takes in the fluid, the liner shrinks down, so the baby doesn’t take in a lot of air bubbles.

@Jeruba, yes, but the “bottle” stays the same size, and I think it’s pretty small volume, and not super resilient.

But, yes on the order of that concept, for adults.

I was thinking something might exist already.

Bota bags, fabric drinking bags, and plastic drinking bags all lose volume as their internal contents are consumed.

Although their surface areas remain the same, the space they occupy shrinks.

I suppose balloons fall in the bag category.
There are those expandable garden hoses. We have one. Amazing.

I used to have ‘shrinkable’ plastic containers when I had a darkroom.
They held about a liter, maybe a liter and a half, and they were harmonica shaped (well, bottle shaped, with harmonica formed material as the body).
The less fluid was in, the lower you pressed the top towards the bottom (I suppose to be left with as little air as possible).
Maybe you can still buy those.
Yes, you can.

Silicone collapsible travel cup. https://www.amazon.com/Lightweight-Collapsible-Cup-Lanyard-Portable/dp/B0777L26PL I’ve never tried it. I do have other collapsible kitchen items. I’d look for something in a local store rather than buy online.

Or, use two short thermoses if you can store one and carry one or carry both when needed.

@rebbel That looks great. Sturdy. The only problem is the shape at the top would make it hard to drink out of. Bottle shapes usually are curved at the top when it’s a small screw top to drink out of. If the OP is just pouring the fluid into a glass it looks really good. I’m glad to know about this bottle for other things.

@rebbel yes!

I wonder if it’s a non-toxic material.

I found what I want. It’s called Hydaway.

Thanks everyone.

@rebbel I think you mean accordion. XD

I use a system called Platypus on my backpacking trips. They weigh nothing empty and can hold lots of water. It’s not insulated, though.