This is a copy of the issue submitted by @mrextreme at the jQuery Mobile repo:
jquery-archive/jquery-mobile#5874

I just took a look at the minified CSS the ThemeRoller creates after an upgrade (from an 1.3.0 file to a 1.3.1), and it is sub-optimal, to say. It is FULL of whitespaces.

A few examples:

border:1px solid #B3B3B3 ;
text-shadow:0  1px  1px  #333333 ;
background-image:   -moz-linear-gradient( #a6ce39,#72bf44 ); background-image: .....
background-image:        linear-gradient( #FFa621,#FF9000 );

Spaces are everywhere!

I ran the good ol' YUI Compressor on the already "minified" file, and from 21.3 kB it went down to 19.8. (I stripped off the JQM header by hand beforehand, as it has nothing to do with this experiment, and should stay in the file.)