If you have ever tried to get isotonic or hydration powder through the neck of a bottle or bag in a hurry then you will appreciate why Platypus developed the Big Zip!  The whole width of the bag opens to get powder (and water) into it, then seals by squeezing it between thumb and forefinger.  The Big Zip also means that it will dry faster after washing.

The bag measures 16cm. x 44cm.   As there is no cap, the tube attaches straight to a welded connector on the front of the bag