I suppose it's more of a concern in multiplayer servers. The server I play on typically has at least 3-4 of us, and early one we still struggle with food and water in A20.
That said, given the apparent size of the dew collectors vs how much water they output.. they seem a little big. a 3x3x4 item should be generating more than just 3 bottles of water per day. If they want to keep it to three per day, fine, but given that the model is simple a plastic barrel and an umbrella, 2x2x2 seems more reasonable.