wow, that is a huge size for a thumbnail that is at 40% quality. I cant really think of what could do it, I know outdoor pictures always seem to be twice as large as the rest (trees and such take alot of space). Besides that, it may just be the program...
I never used infranview, it may work great.
But in my opinion, if you have a little extra cash, get "compupic pro" - ive used it for years to make my thumbs, crop pics, and just organize everything. you can get a free trial of it i believe,
http://www.photodex.com/downloads/trials.html
cant remember if it leaves a watermark or not on the trial, worth a try though! Out of the 3 or 4 programms ive used, compupic is by far my favorite, their batch conversions can do loads of things all in one swipe
Sorry thats about all I can suggest, some programs are just really bad at optimizing images...