I've heard a rumour, a strong one mind, that they're not the only Cosworth powered team to have undersized tanks and that this other one really should know better! Apparently, again unsubstantiated rumour, they were supplied fuel economy data that has proved to be slightly optimistic and will have to run the first 10 or so laps in fuel-saver mode. They'll be running very lean, hot, and slightly underpowered.
To be fair, the casual observer (myself included) will probably not notice this in the general melée and tight formation of the opening laps.