Last modified: 2013-04-22 16:16:25 UTC
The HTML output for a thumbnail image refers to the original, full-size image rather than a scaled image. This means that if you open [[Golden Gate Bridge]] in the editor, it proceeds to download a 12 megapixel, 8 MB picture of traffic on the bridge. In the actual article, MW uses a scaled-down 220x331px version which is only 32 KB. And there are quite a few images on that page, so this adds up :)
Patch pending in https://gerrit.wikimedia.org/r/34208
Merged, so closing as fixed.