Well, without having read the whole article, it seems to me that the reason they're getting this result is that we're comparing bureaucrats to the general public.
The general public doesn't know how much is actually spent. They vastly overestimate spending on foreign aid, for instance, and welfare. It's plausible that bureaucrats prefer levels of spending that are roughly commensurate with actual operating costs, while ordinary citizens are just mis-estimating.
The fact that the public doesn't know the actual budget explains several strange artifacts in polling. For instance, when you poll people, a large percentage think foreign aid should be cut, but when you ask them what percent of the budget it should be, it's still higher than the actual percent! There's a similar phenomenon with taxes: people tend to think they should be cut, but they also overestimate how high taxes are in the first place, so they sound like they're calling for higher taxes.
It might be interesting to have two versions of a poll on government spending, one of which tells the participants the current spending levels before asking the questions, and compare the results.
Jerry Pournelle's "Iron Law of Bureaucracy" implies that leaders of bureaucratic organizations will seek to maximize the power and influence of the organization at the expense of its stated goals - but is that true in the real world?
Julie Dolan of Macalester College examined surveys of government administrators and found that, surprisingly enough, high-ranking federal bureaucrats tended to prefer less government spending than the general public, even on issues that their own departments are responsible for.
Here is the abstract from her paper, "The budget-minimizing bureaucrat? Empirical evidence from the senior executive service" that was published in the journal Public Administration Review:
I was able to read the paper here for free, but I had to register first.
See also: The Case FOR Bureaucracy