\documentclass{article}
\usepackage{amsmath}
\usepackage{lmodern}
\DeclareFontShape{OMX}{cmex}{m}{n}{%
<-7.5> cmex7
<7.5-8.5> cmex8
<8.5-9.5> cmex9
<9.5-> cmex10
}{}
\DeclareSymbolFont{largesymbols}{OMX}{cmex}{m}{n}
\begin{document}
\scriptsize $\displaystyle\sum\int$\par
\footnotesize $\displaystyle\sum\int$\par
\small $\displaystyle\sum\int$\par
\normalsize $\displaystyle\sum\int$\par
\large $\displaystyle\sum\int$\par
\Large $\displaystyle\sum\int$\par
\LARGE $\displaystyle\sum\int$\par