Key to make great thumbnails. Next, if you don't need animation or transparency, you'll get smaller file sizes with jpg than gif in most cases. Save either way and check file sizes. With jpg, you can adjust compression rates. Again, you got to try.
Results depend on the structure of the original.
Your thumbs are way too fat. They weigh in for about 40 KB and should have at most 10. No visible difference to the original. If you like to see the proof, I'll send them attached. Oh, I see, you only have the blowup and reduce that to a thumb.