com.ibm.wsspi.jsp.usecdatatrimcustom property to true for your JSP engine, all of the white space and extra lines in the generated Java code are stripped out.
Take a look at the page which has two portlets and default IBM theme,
The page that gets generated is 64kb's in size and if you look at the view source you can see all the white spaces.
com.ibm.wsspi.jsp.usecdatatrimequal to true as Web Container Custom property like this
Restart the server and try accessing the same page, you will notice that the size of the markup is reduced to 56.4 kb (Which is saving of close to 15 %) like this
The saving size could be much larger if your page is more complex.