Well a lot of it depends on the image format, the main 3 being .jpg, .png, and .gif.
JPGs are typically used for photos; images with lots and lots of colors and complex color boundaries etc... They are lossy, meaning when you save, you will lose some quality and get artifacts (nasty looking pixely areas), but have pretty decent file sizes (though that depends on how much quality you're willing to sacrifice)
PNGs are usually used for computer graphics, such as gradients, icons, translucent/transparent images... They have really good file sizes for what I listed, for images, not so much. They are non-lossy, so you will not lose quality and get those artifacts.
GIFs are useful (and small) for simple images that don't have many colors, up to 256 I believe. PNGs can do everything GIFs can (except animation) and more (alpha transparency)
Your jpg images aren't that large (150kb), especially now in 2010, where lots of ppl have high speed internet, although compressing and optimizing images is always a good idea. So the "key" to keeping file size down is to use the proper image format and compress and sacrifice as little quality as you are willing.
The images that were around 90kb likely had lots of one color (sky etc...), whereas yours have lots of complex patterns (tree branches etc..) This image
is only 70kb, which makes sense, as most of the image is a green sky and some black cave.
I think the compression gives your images a nice nitty gritty look anyway.
Hope that answered your question