Dunno what you are looking at, but the unminified jquery.mobile-1.2.0.css is 109K, and the minified is 72.8K. That's far from 147K. Maybe you have some unneeded files, or perhaps you are counting your own CSS overrides as well.
If you use a theme file, then the jquery.mobile.structure-1.2.0.css is 53.4K unminified and 47.3K minified (it doesn't minify as well as the theme). The theme comes in at 53.2/25.7.
You can save some if you don't need all of the default themes.
You can save some by using the download builder and eliminating unused features. This will in some cases remove some of the CSS. (Haven't experimented with this, though.)