I am having a problem similar to this and this, but without using parskip, and only after a recent package update. The MWE
\documentclass{scrartcl}
\usepackage{amsmath,amsthm}
\theoremstyle{plain}
\newtheorem{thm}{Theorem}
\begin{document}
Some text before the theorem.
\begin{thm}
Fun with theorems.
\end{thm}
Some text afterwards.
\end{document}
produces the following, certainly wrong spacing:

The space between the theorem and the preceding and succeeding text has gone. This only happens with scrartcl, not with article.
fontsizeby themselves. – Deniz Jan 02 '15 at 09:10fontsizemechanism runs pretty deep. I had limited success with the first option, but the second (fixing thescrsizefiles) worked for me. – Christian Clason Jan 02 '15 at 11:22