Consider the following four representations of the same mathematical expression.

The first row is from your posting. TeX knows about scriptstyle (for first-level subscripts and superscripts) and scriptscriptstyle (for second-level subscripts and superscripts) math, but not about scriptscriptscriptstyle. That's why the letter B
looks too big relative to k
and T
.
The second row succeeds in making the overall expression less tall, by replacing \frac{1}{...}
with [...]^{-1}
. However, the letter B
still looks too big.
The third row replaces e^{...}
with \exp(...)
. This change, finally, succeeds in getting the relative sizes about right.
The fourth row replaces frac{a}{b}
with (a)/(b)
, with inline-fraction notation.
In my view, the expressions in rows 3 and 4 both look good.
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\addtolength\jot{4pt}
\begin{align*}
&\frac{1}{1+e^{\frac{\varepsilon-\mu}{k_B T}}} \\
&\Bigl[1+e^{\frac{\varepsilon-\mu}{k_B T}}\Bigr]^{-1} \\
&\Bigl[1+\exp\Bigl(\frac{\varepsilon-\mu}{k_B T}\Bigr)\Bigr]^{-1} \\
&\bigl[1+\exp\bigl((\varepsilon-\mu)/(k_B T)\bigr)\bigr]^{-1}
\end{align*}
\end{document}