Oxygen reacts with the developer's salts (metol, sulphite, etc) and they become other salts.
Sulpite, if I'm not wrong, becomes sulphate - and useless as an anti oxydizer (but then it's there, among lots of other reasons, to protect the real developer agent's by absorbing oxygen).
This way, it occupies very much less volume.
It's quite well known the fact that a developer's bottle (tight closed) may look 'crushed' when partially filled if it's not strong enough. Try it!
Re brown bottles - they may not be good for long term storage, but I've never had problems with say one month.
Now, Bob's point re film developer in glass bottles makes a lot of sense.