How are you saving the image? You're using 'Save for Web', right? If so, try messing around between .gif, .jpeg, and .png to see which gets you the best file size for the best quality. I save most of my stuff as .png, it just seems to work better all around. And do check dpi... I can't remember what I work in right now, or I'd tell you.

'Save for Web' is the one you want. However, I vaguely recall in 7.0 that sometimes it'd just refuse to obey, so you might want to upgrade to CS2, OR, try saving files through Imageready, which should be on your comp if you've installed Photoshop.

Another thing that might might be the problem but likely isn't is that you're seeing a difference in actual file size and size on disk, however let's not attempt to check that until you exhaust a few other possibilities first.

Like Yasha said, jpeg is rarely the best option for small images anyway, especially images like anime. Manga would suit jpeg usually but block color is done much better by png. That said I don't remember if PS7 and below has that as a function, but if you save it first as a gif, you can convert to png (and see how much size you shaved off!) at this nifty site. http://www.r1ch.net/stuff/gif2png/webinterface/

But first let us know if you're using Save for Web, which I think you are; so try Imageready first. I recall PS7 being a bit buggy this way, if Imageready doesn't work, try uninstalling and reinstalling. A lot of weird random solutions done all at once fixed it for me, but this was a long time ago. Your best bet if your computer can handle it is upgrading Photoshop. (CS2 has a lot of great features, too!)