Unfortunately phantomjs produces different font sizes when being run on windows vs linux. Here you can find some notes in docs together with style based workaround which may work for you.
https://jsreport.net/learn/phantom-pdf#different-sizes-on-windows-vs-unix
Another workaround using css zoom is mentioned here
https://forum.jsreport.net/topic/188/dpi-option-doesn-t-seems-to-work/6