This is kind of a black box in the Glowforge software.
First, what’s going on, your PNG is defined as X*Y pixels (say, 500x500 pixels). The browser has its own interpretation of Pixels per Inch (96). So, when you upload a 500x500px image, it’s taking that and interpreting 500/96 as the size of the image. When you get into high-resolution images - say you exported a 5x5” image at 300 PPI, your image is now 1500x1500, but when you upload it to the app, it will size itself to 15.625” instead of 5x5” @ 300 PPI.
The black box side of it is what happens to the image when you scale it back to 5x5”? Is it increasing the PPI as you scale the image down? Is it keeping it at 96 PPI? 
What one can do though, is embed your hi-res raster images into a SVG file. It will maintain the correct size when you upload the SVG. I’ve also seen a noticeable difference in testing these prints when embedding 72, 300, 600 PPI images into the SVG.