Costs don't scale linearly with pageviews. Nor do
donations,
especially when you consider that much of that growth in pageviews now
comes from the 'Global South' (where people generally have less
disposable income to donate) and from mobile devices (which we don't
really fundraise on, although I believe this is something WMF wants to
work on next year).
This very reasonable sounding theory is contradicted by measuring the
actual data at
http://wikimediafoundation.org/wiki/Special:FundraiserStatistics
I have heard so many reasonable sounding theories from Foundation
staff who refuse to measure the corresponding data over the past year
that I am beginning to wonder whether I should compile them for
publication.