\begin{table}[htbp]
\begin{center}
-\begin{changemargin}{-2.5cm}{0cm}
+\begin{changemargin}{-1.8cm}{0cm}
+\begin{small}
\begin{tabular}{|c|c||c|c|c||c|c|c||c|}
\hline
\multirow{2}{*}{Pb size}&\multirow{2}{*}{Nb. cores} & \multicolumn{3}{c||}{GMRES} & \multicolumn{3}{c||}{Krylov Multisplitting} & \multirow{2}{*}{Ratio}\\
\end{tabular}
\caption{Results}
\label{tab1}
+\end{small}
\end{changemargin}
\end{center}
\end{table}