I closed down FF after that run of FoxySpider and ran CCleaner whilst I made a cuppa tea. When I came back I saw that CC had removed over 2Gb of surplus data...compared to the usual max of 400Mb or so that it would otherwise usually do!
It would be interesting to see if one of those large images, when saved to disk and then opened and re-saved using (say) irfanview, would still be as big on the re-save - even without compression. I recall a discussion a while back on DCF where someone had these humungus image files - wedding photos - that he had recovered from disk. I noticed that after re-saving them in uncompressed form, they were of a "normal" size, and still had the same resolution. The conclusion was that recovered files can contain a load of redundant/surplus data, and I have found that to be consistent with my own experience since in recovering deleted image files from disk.