Text inside Figures appear too tiny with respect to the document and are spaced far apart

I am trying to plot 2 subfigures next to each other, that consist of .pdf graphs, with blank (a) and (b) subcaptions and a main caption at the bottom of the whole figure. However when I output the result, I face two problems:

1. The text within the figures appear far too small with respect to the document font size.
2. The figures themselves are spaced too wide apart with excess whitespace in between

I have tried to adjust the width between the figures and size using \textwidth however it seems that if I try to enlarge the figures for values > 0.4, the figures simply output on top of each other; an undesired result.

So I was wondering if there is any way to address these two problems I am facing without having to edit all my plots at source?

Below is my minimum working example. Here is a folder containing the 2 pdf plots in the image folder: https://drive.google.com/open?id=0B1OA37Nso-fLenMzNnJNRWlHS0k

\documentclass[11pt]{report}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{graphicx}
\usepackage[top=3cm, bottom=2cm,left=3cm,right=3cm]{geometry} % needed for page border settings
\usepackage[english]{babel}
\usepackage{blindtext}

\begin{document}

\begin{figure}[!h]
\centering
\begin{subfigure}[b]{0.4\textwidth}
\includegraphics[width=\textwidth]{images/plot1.pdf}
\caption{}
\label{fig:plot1}
\end{subfigure}
\begin{subfigure}[b]{0.4\textwidth}
\includegraphics[width=\textwidth]{images/plot2.pdf}
\caption{}
\label{fig:plot2}
\end{subfigure}
\caption{This is a caption}
\end{figure}

\blindtext[1]

\end{document}


You can "trim" the pdfs using includegraphics. If all the figures are made the same way, then that is the easiest solution. Of course if your fonts are still too small, you are out of luck, or will have to resort to something complicated enough that simply remaking the figures from scratch will probably be the easiest option.
\begin{figure}[htbp]