Hey cingram, welcome to the forum. First up, which method do you use? Do you use the "Save for Web" feature (Alt+Shift+Ctrl+S)? Do you use the "Save As" method and select JPEG from the drop-down menu? Also do you have "Image Preview" set to on in your preferences?

There are a few factors that impact the file size in your thumbnails and colour range is one, especially if the are subtle shifts like gradients and smoke or mist. These all add to the number of colours used. One trick i do is to use the "Sharpen" filter to tighten up some detail lost when resizing the thumbnail.

:)
retrocity