The issue is that the size of the SVG file is specified as 500 by 500 pixels. When opened in Inkscape, it looks pretty much like the preview (and the result) in GIMP:
A look at the Document Properties confirms this: the size of the document is 500 by 500 pixels:
This doesn't prevent the file from being brought to any desired size, of course - but you'd have to specify a pretty large image size to get the visible parts to a reasonable size, too. A better approach is to change the document size to the actual drawing, by the button shown in the previous screenshot:
Voilà! The SVG file's width and height are now much smaller, and it is much easier to enlarge the visible parts without dealing with all the extra area. You still have to specify a larger size to get a useful image in GIMP, but you will likely find it much easier to understand what e.g. doubling the size results in.