I believe the plugin that creates these images uses a script to automatically size the image based on viewing settings (eg- browser window size), hence there are a few approaches you can take to get bigger images.
1. Create the image from scratch (more work, most custom) - go to the download tab of the expression viewer to download the data, and create a horizontal bar graph with it in Excel
2. Maximize the display of the image in your browser before saving it. First, open the frame containing the expression viewer in a new tab or window (so it's by itself and not constrained by other plugin content). Maximize, that browser window, and then save the image. It should give you a bigger image. You can then adjust the resolution by resizing your image without resampling it in a simple image viewer/editor like Irfanview.
3. User your browser's built in web dev tools to trick the page into thinking you have a higher res screen/bigger browser. Here are the instructions in firefox, and it should be similar for Chrome...No idea about Safari though.
A. open the frame in a new window (right click in the correct frame).
B. Go to Tools>>Web developer>>Responsive Design Mode.
C. Change the resolution to something bigger. (In this case, I've changed it to 2000x1000)
D. Right-click and save the image. You should now have a bigger image. You can then use an image editor to resize it to the appropriate resolution, and resample it to the desired size.