Chrome's Device Dimensions confuses Nimbus Capture
Chrome has an option to resize the browser viewport to some typical device size. When this option is used, Nimbus Capture generates a screenshot with the correct size, but with the actual screen contents too small.
For example, in the first screenshot below, the size I chose is 1024x768 and this is the size of the image that Nimbus Capture generated, but the actual contents only appear in about the top-left quadrant of the image. You can check by right-clicking the image in this page, and opening the image separately.
I expect the contents to fill the image canvas, as per the second screenshot which I took with a different tool.