Measuring jug, in inventory, in backpack presents the "Set Volume To" menu, but when a volume is selected, reports:
Possible bug: The measuring jug does not seem to be able to detect that it is inventory, when in a backpack.
Alternatively, if this is the desired behavior (?!), the message is confusing, since the measuring jug is in inventory (in a backpack). In that case, the message should read something along the lines of "You can only change volume when the measuring jug is in your inventory and not in any container." Although I have not tested that this issue occurs in other containers.
Workaround: move the measuring jug out of backpack, change its volume, return to backpack.