Glancing at the docs you reference, roughly $6M of their spending goes to straight operating expenses (servers, bandwidth, travel, etc). I don't think anyone would disagree that is completely reasonable for an organization Wikimedia's size with the amount of traffic they have (it seems quite lean, even compared to most startups).
The other big chunk ($3.5M) is salary for people.
There you have to take into account that they have ~70M unique users a month. Assuming $150K/engineer/year all told (office space, computers, salary, benefits, etc) and that all the salary were just going to engineering, that's only enough cash to hire about 23 engineers (or about 1 engineer for every 3M users). In practice, not all their employees are engineers, so it's probably closer to 5M users per engineer. As a point of comparison, Facebook brags about having only 1 engineer per million users.
Wikipedia is being laudably frugal and efficient. I can't imagine running a site their size with much fewer resources.