A couple of days ago, in Google Webmaster Tools, kilobytes downloaded per day skyrocketed like 3X, while pages crawled per day remained consistant. It's been like this for a couple of days now. I haven't make any big changes that I can think of that would cause this type of behavior ... I haven't suddenly added a lot of extra images or resources or doubled the average length of a page or anything like that.

has the site providing ad services changed their format
the ad script is included in the KB count of the page, and you dont have much control of that
when developer comments and keys are not removed from the first production run of code,
:token: //this is 20kb to explain how we reduced the filesize by tokenizing

has the site providing facebook services .. ditto the above