Changing the font size in koma-script modifies page margins

Question

I'd like to use the koma-script scrartcl class with a font size of 10pt, but when I use the document option 10pt, not only the font size decreases but also the space user for the text. Especially the footer margin (between the page number and the bottom of the page) is enlarged and this looks unpleasant to me.

Minimal document with 11pt font size:

\documentclass[a4paper]{scrartcl}
\usepackage{lipsum}
\begin{document}
\lipsum[1-20]
\end{document}

Minimal document with option 10pt (and large footer margin):

\documentclass[a4paper,10pt]{scrartcl}
\usepackage{lipsum}
\begin{document}
\lipsum[1-20]
\end{document}

Why does changing the document font size affect the margins so much? How can I set the font size to 10pt and not influence the margins of the page?

My system is configured to use a paper size of A4 by default and I'm compiling my documents with pdflatex.

Answer 1

The KOMA-Script classes uses DIV=default as default option for the page layout. default for a4paper and 11pt means 10. But with 10pt and a4paper default is only 8 resulting in bigger margins.

If you really want you can set DIV=10, but you will get a warning. Maybe DIV=9 is a compromise:

\documentclass[10pt,DIV=9]{scrartcl}
\usepackage{blindtext}
\begin{document}
\blindtext[6]
\end{document}

Answer 2

The documentation of KOMAscript has a whole chapter about how the text area is calculated according to typographic principles.

Basically it boils down to certain relations between text area and blank spaces as well as aspect ratios of paper and text area. But also the font size plays a role as there is a (more or less strict) optimum of how many characters should be in a line for good readability. If you change font size you will have to adjust the width of the text area and therefore also the page margins to account for the change in line length.