That is just strange. I took down the image, obviously it is a jpg now, full 100% in save for the web is 35.65k, when I convert it to med/30% it is 12.48k.
Go to File > Save for Web. A window will pop open and there are a whole slew of possibilities in there. You can set the % of the jpg file you want. It is also the place to save as png and gif, you can loop your animations as well.
The nice thing is that within this window you will see how the original psd file looks and the comparison when the file gets compressed
leopard, you might find the following article useful, it has some additional optimization tips which could help you squeeze some extra KB outta your images:
what’s the difference between this and just reducing the image quality and saving again?
still trying to understand what’s better - high dpi and small size or vis verse