The theme with smaller fonts is called normsm*.css.dsp. You can configure WEB-INF/zk.xml to use it as follows.

<desktop-config><disable-theme-uri>~./zul/css/norm*.css.dsp*</disable-theme-uri><theme-uri>~./zul/css/normsm*.css.dsp*</theme-uri></desktop-config>        

Refer to the Developer's Reference for more about how to configure with WEB-INF/zk.xml.