"Beautifully" resize math align block (font and symbols)

Question

I'm using \mathlarger from relsize package to get larger fonts in my beamer presentation (in some VERY special cases, not to all equations). But the symbols are not scaling correctly, I'm mostly worried about the integral symbol. For example:

\documentclass{beamer}
\usepackage{lmodern}
\usepackage{relsize}
\usepackage{amsfonts}
\usepackage{amsmath}
\usepackage{amssymb}

\begin{document}

\begin{frame}

\begin{align*}
\int_0^1 \frac{d}{dx} f(x) dx = f(1) - f(0)
\end{align*}

\begin{align*}
\mathlarger{\mathlarger{\int_0^1 \frac{d}{dx} f(x) dx = f(1) - f(0)}}
\end{align*}

\begin{landscape}
     \begin{center}
        \resizebox{7cm}{!}{$\int_0^1 \frac{d}{dx} f(x) dx = f(1) - f(0)$}
     \end{center}
\end{landscape}
\end{frame}

\end{document}

As proof of concept

First: regular; second: \mathlarger; third: \resizebox

Just the regular has the nicely fit integral symbol

Answer 1

This works as expected:

\documentclass{beamer}
\usepackage{lmodern}
\usepackage{relsize}
\usepackage{amsfonts}
\usepackage{amsmath}
\usepackage{amssymb}

\DeclareFontFamily{OMX}{lmex}{}
\DeclareFontShape{OMX}{lmex}{m}{n}{%
   <->lmex10%
   }{}


\begin{document}

\begin{frame}

\begin{align*}
\int_0^1 \frac{d}{dx} f(x) dx = f(1) - f(0)
\end{align*}

\begin{align*}
\mathlarger{\mathlarger{\int_0^1 \frac{d}{dx} f(x) dx = f(1) - f(0)}}
\end{align*}

\end{frame}

\end{document}

The issue arises because the omxlmex.fd file, which tells TeX where to look to find the TFM file for the lmex family (the family that the lmodern package uses as a replacement for the cmex one — in other words, for the “large symbols) loads the cmex10.tfm file at a fixed size equal to the design size. Actually, the omxcmex.fd prescribe exactly the same behavior for the cmex family, but the amsfonts package (and also, albeit to a limited extent, the amsmath package) add a patch to correct this. The amsfonts (and amsmath) packages do not care about the lmex family, though: this is why one needs to introduce the corresponding patch by hand, when the lmodern package is used. This is exactly what the lines

\DeclareFontFamily{OMX}{lmex}{}
\DeclareFontShape{OMX}{lmex}{m}{n}{%
   <->lmex10%
   }{}

do.