Sounds like you’re getting bitten by the renditions bug that was introduced by Adobe some months ago. We posted updates, of course, and sent out a newsletter with the subject “IMPORTANT”.
In short, Adobe made a breaking change in their API, and older versions of publisher no longer respect your width/height settings for renditions. The impact is that publisher now exports thumbnails as full-size images. Not good.
We fixed this in Backlight 2 and newer. We very strongly advise an upgrade.
If you’re intent on trucking on with Backlight 1, though, then see here: