Issue
When I set my Netbeans look and feel to nimbus and a small font seems great .
But when I set the font size to 14(or bigger than) some size of texts are OK but others will be made smaller than like size 10(though I set the font size to 14).
the covered areas with green mean that the font within them are set great.
the covered areas with red mean than the font within them are set incorrect (smaller than gotten value)
Gtk THEME:
Nimbus Look And Feel : 
OS : Linux Manjaro
Jdk : open jdk 14
Environment : GNOME
Netbeans : Apache Netbeans 12
"It probably happens only on Linux and might be a java SE problem not a Netbeans problem"
Solution
It seems that the problem is within the Java on linux as it has some problem with javafx media players in linux as well.
Also as I tested other look and feels the only problem is in the nimbus and I have to report it to Oracle.
Answered By - user13226212