Thanks for your response, DavidR:

After noticing this problem for weeks (months?) I can say it appears to be suddenly resolved, due to no particular changes made by me: not to avast nor to IE8 settings.

I have had the Avast Online Security Add-on disabled in IE8 ever since I installed Avast 2014 weeks ago.

Like you, my TIF files now only number in the 300-400 KB range (according to CCleaner), and the folder contains only protected (login) cookies and an icon or two.

Very curious that this glitch should suddenly disappear shortly after my original post.

Thanks again.