Font sizes when sending book from Calibre to Kobo

I wanted to have a standard size and font for my books both in Calibre and when sent to my Kobo.
I set the font size in: Preferences>Conversion>Common Option>Fonts to 13pt. I then converted a small selection of books EPUB to EPUB. Using the Calibre ebook viewer the fonts appeared to be the same size. However when sent to my Kobo there was a significant difference between font sizes.

Am I doing something wrong here and is there a way to have my Kobo reader display a set font size, line spacing and margins.

Any advice much appreciated.