It is true that different languages display the fonts differently.
I recommend that you change the css file for Greek, but not for English.

Go to the directory:
/public_html/includes/templates/your-override-directory/css.
Here, you can create files named:
english_stylesheet.css
greek_stylesheet.css
Just copy the css/sytlesheet.css file into this folder twice and rename it to these.

Leave the English one alone if you like how English displays, and change the Greek one to font sizes that look good in Greek.