There are many people on this board and even in this particular thread who know much more about chemistry than I do, but I'll take a stab at answering this anyway.
When you buy a batch of Xtol you'll see that it comes as a loose powder in a package (actually two packages) that is supposed to be mixed with a specified amount of water (5 liters, in the case of Xtol). The powder is composed of several separate components -- i.e. different chemicals -- whose ratios are very carefully established by the manufacturer. So as I have always understood it, if you tried to take, say, half the powder and mix it with 2.5 liters of water, you can't guarantee that you get exactly half of all the different chemicals that are in the powder. They may not be distributed evenly in the powder. That's been my assumption, at least.
I'm sure people have tried mixing half the powder, or a quarter of it, because human beings do such things, but I've never heard of anyone doing it successfully. Everyone I'm aware of mixes the whole package of powder at one time. Several well-known powdered developers are available in smaller batches -- i.e. enough powder to make one liter. That's less of a storage burden, obviously. But Xtol is no longer available in that smaller size. (A similar developer from Adox, XT-3, is available in packets to make one liter, I believe.)
This is a big difference between powdered chemicals and liquid chemicals. With liquids, you can measure out a small portion of it -- i.e. just the amount you need at that moment -- and mix it with water, then develop your film or paper. The logical guess for a non-chemist like myself is that in liquid form, the separate component chemicals are distributed evenly within the liquid.
As for storage bottles, you can get both dark (usually black or brown) or white plastic bottles (and maybe other colors?) in various sizes from photography retailers. Freestyle Photo, which is a sponsor of this site I believe, sells them. A lot of photographers also use brown glass bottles. Here's Freestyle's bottle page:
Bottles at Freestyle Photo & Imaging Supplies
www.freestylephoto.biz