Here are three different versions of your two subtables:
For the first version, I have used \captionsetup[subfloat]{width=\textwidth} in order to make the subtable's captions use the whole width instead of just the width of the corresponding subtable.
In the second example, I have moved repeated information from the subtable's captions to the main caption.

In the third table, I have merged both subtables into a single table to save some space. I have also used the booktabs package for the horizontal lines and removed all vertical lines:
\documentclass[review,review,12pt]{elsarticle} \usepackage{subfig} \captionsetup[subfloat]{width=\textwidth} % To use the full width of the page instead of the width of the corresponding subtable. % only used for last example \usepackage{booktabs} \usepackage{makecell} \renewcommand{\theadfont}{\normalsize} \begin{document} \begin{table} \centering \caption{Use of added entries by agencies \label{addedEntriesAgencies}} \subfloat[Difference in registering added entries between agencies (across 5815 books cataloged by both agencies) - total occurrences of sub-fields]{ \begin{tabular}{|| c | c | c | c | c || } \hline Agency & 700 $\$a$ & 700 $\$e$ & 700 $\$t$ & 740 $\$a$\\ \hline BIBBI & 5844 & 4574 & 1272 & 775\\ Bokbasen & 5632 & 5623 & 0 & 693\\ \hline \end{tabular} } \subfloat[Difference in registering added entries between agencies (across 5815 books cataloged by both agencies) - Records with at least one occurrence of sub-field]{ \begin{tabular}{|| c | c | c | c | c || } \hline Agency & 700 $\$a$ & 700 $\$e$ & 700 $\$t$ & 740 $\$a$\\ \hline BIBBI & 3354 & 3262 & 179 & 571\\ Bokbasen & 3667 & 3664 & 0 & 417\\ \hline \end{tabular} } \end{table} \begin{table} \centering \caption{Use of added entries by agencies -- Difference in registering added entries between agencies (across 5815 books cataloged by both agencies) \label{addedEntriesAgencies}} \subfloat[total occurrences of sub-fields]{ \begin{tabular}{|| c | c | c | c | c || } \hline Agency & 700 $\$a$ & 700 $\$e$ & 700 $\$t$ & 740 $\$a$\\ \hline BIBBI & 5844 & 4574 & 1272 & 775\\ Bokbasen & 5632 & 5623 & 0 & 693\\ \hline \end{tabular} } \subfloat[Records with at least one occurrence of sub-field]{ \begin{tabular}{|| c | c | c | c | c || } \hline Agency & 700 $\$a$ & 700 $\$e$ & 700 $\$t$ & 740 $\$a$\\ \hline BIBBI & 3354 & 3262 & 179 & 571\\ Bokbasen & 3667 & 3664 & 0 & 417\\ \hline \end{tabular} } \end{table} \begin{table} \centering \setlength{\tabcolsep}{4pt} \caption{Use of added entries by agencies -- Difference in registering added entries between agencies (across 5815 books cataloged by both agencies) \label{addedEntriesAgencies}} \begin{tabular}{l*{8}{r}} \toprule & \multicolumn{4}{l}{\small a) total occurrences of sub-fields} & \multicolumn{4}{p{5cm}}{\small b) Records with at least one occurrence of sub-field}\\ \cmidrule(r){2-5} \cmidrule(l){6-9} Agency & 700 $\$a$ & 700 $\$e$ & 700 $\$t$ & 740 $\$a$ & 700 $\$a$ & 700 $\$e$ & 700 $\$t$ & 740 $\$a$ \\ \midrule BIBBI & 5844 & 4574 & 1272 & 775 & 3354 & 3262 & 179 & 571\\ Bokbasen & 5632 & 5623 & 0 & 693 & 3667 & 3664 & 0 & 417\\ \bottomrule \end{tabular} \end{table} \end{document} \documentclass[review,review,12pt]{elsarticle} \usepackage{subfig} \captionsetup[subfloat]{width=\textwidth} % To use the full width of the page instead of the width of the corresponding subtable. % only used for ast example \usepackage{booktabs} \usepackage{makecell} \renewcommand{\theadfont}{\normalsize} \begin{document} \begin{table} \centering \caption{Use of added entries by agencies \label{addedEntriesAgencies}} \subfloat[Difference in registering added entries between agencies (across 5815 books cataloged by both agencies) - total occurrences of sub-fields]{ \begin{tabular}{|| c | c | c | c | c || } \hline Agency & 700 $\$a$ & 700 $\$e$ & 700 $\$t$ & 740 $\$a$\\ \hline BIBBI & 5844 & 4574 & 1272 & 775\\ Bokbasen & 5632 & 5623 & 0 & 693\\ \hline \end{tabular} } \subfloat[Difference in registering added entries between agencies (across 5815 books cataloged by both agencies) - Records with at least one occurrence of sub-field]{ \begin{tabular}{|| c | c | c | c | c || } \hline Agency & 700 $\$a$ & 700 $\$e$ & 700 $\$t$ & 740 $\$a$\\ \hline BIBBI & 3354 & 3262 & 179 & 571\\ Bokbasen & 3667 & 3664 & 0 & 417\\ \hline \end{tabular} } \end{table} \begin{table} \centering \caption{Use of added entries by agencies -- Difference in registering added entries between agencies (across 5815 books cataloged by both agencies) \label{addedEntriesAgencies}} \subfloat[total occurrences of sub-fields]{ \begin{tabular}{|| c | c | c | c | c || } \hline Agency & 700 $\$a$ & 700 $\$e$ & 700 $\$t$ & 740 $\$a$\\ \hline BIBBI & 5844 & 4574 & 1272 & 775\\ Bokbasen & 5632 & 5623 & 0 & 693\\ \hline \end{tabular} } \subfloat[Records with at least one occurrence of sub-field]{ \begin{tabular}{|| c | c | c | c | c || } \hline Agency & 700 $\$a$ & 700 $\$e$ & 700 $\$t$ & 740 $\$a$\\ \hline BIBBI & 3354 & 3262 & 179 & 571\\ Bokbasen & 3667 & 3664 & 0 & 417\\ \hline \end{tabular} } \end{table} \begin{table} \centering \setlength{\tabcolsep}{4pt} \caption{Use of added entries by agencies -- Difference in registering added entries between agencies (across 5815 books cataloged by both agencies) \label{addedEntriesAgencies}} \begin{tabular}{l*{8}{r}} \toprule & \multicolumn{4}{l}{\small a) total occurrences of sub-fields} & \multicolumn{4}{p{5cm}}{\small b) Records with at least one occurrence of sub-field}\\ \cmidrule(r){2-5} \cmidrule(l){6-9} Agency & 700 $\$a$ & 700 $\$e$ & 700 $\$t$ & 740 $\$a$ & 700 $\$a$ & 700 $\$e$ & 700 $\$t$ & 740 $\$a$ \\ \midrule BIBBI & 5844 & 4574 & 1272 & 775 & 3354 & 3262 & 179 & 571\\ Bokbasen & 5632 & 5623 & 0 & 693 & 3667 & 3664 & 0 & 417\\ \bottomrule \end{tabular} \end{table} \end{document}
subfigurepackage is deprecated. Use the environments from thesubfigpackage (that you also already load) instead.