Sorry, it is the second post from @Jules that should help you.
It has to do with your settings for saving an SVG. I use Inkscape, so I am not an Illustrator guy, but I do recall that this has been a constant source of frustration to folks new to designing in Illustrator.
If you think of a vector design program that can make objects that can scale perfectly in any viewable media. Something has to give it a reference scale to ensure that it remains true to the intended scale.
A design program has to think of scales in web browsers and many other display situations. Computer screens have pixels and one counts the pixels per inch. Some monitors have different pixels per inch. You can even set your display to resolve and image differently.
That is why you have to set the saved SVG with a specific reference size to ensure that the SVG is read correctly in the Glowforge user interface.