with use of mathtools and nccmath you can slightly reduce size of equation (without changing font size):

\documentclass[hyperref={pdfpagelabels=false}]{beamer} \usepackage{mathtools, % instead amsmath, provide `\mathclap` nccmath, % for smaller `sum` symbol amssymb} \begin{document} \begin{frame}{test} \begin{align} & \textbf{TP} ~~\max \medop\sum_{\mathclap{i \in N\setminus\{4\}}} J_{i} z_{i} \label{S1}\\[-1ex] & \text{st} \nonumber\\ & \medop\sum_{j \in N} xv_{ij} = z_{i}, ~~\forall i \in N \label{S2} \end{align} \end{frame} \end{document} 

note: eqnarray is long time deprecated and replaced with math environment from amsmath.

for comparison result with your mwe:

with use of mathtools and nccmath you can slightly reduce size of equation (without changing font size):

\documentclass[hyperref={pdfpagelabels=false}]{beamer} \usepackage{mathtools, % instead amsmath, provide `\mathclap` nccmath, % for smaller `sum` symbol amssymb} \begin{document} \begin{frame}{test} \begin{align} & \textbf{TP} ~~\max \medop\sum_{\mathclap{i \in N\setminus\{4\}}} J_{i} z_{i} \label{S1}\\ & \text{st }   \medop\sum_{j \in N} xv_{ij} = z_{i},\ \forall i \in N \label{S2} \end{align} \end{frame} \end{document} 

note: eqnarray is long time deprecated and replaced with math environment from amsmath.

for comparison result with your mwe: