@clndwhr I always recommend using my Grbl 1.1e Default firmware, the motion is faster and smoother compared to 0.9 and you'll get features like real time overrides, dynamic laser power and more. I also include homing in my firmware should you wish to add switches later.
So anyway, back to the real question. If I understand correctly you noticed that when tracing the size isn't exactly matching the original image. Usually the trace is exactly the same size although it's possible there could be a small error. The way auto-tracing works is it finds a contrast change and then follows that line, so say it detects a change from black to white, it will stay on the black pixel side and continue around until it makes a shape. This offset is 1 pixel so at 0.15 resolution you'll add 0.3mm (1 pixel on each side) or if it was tracing the other side you'd lose the same size. It should not multiply though so you should never lose more than 2 x the resolution which is generally very small.
Since you are seeing a much larger offset and it's not within the image but the overall size, there could possible be something else going on. Have you checked the distance calibration and adjusted if needed? This option is on the laser controller screen in the settings menu.
Do you have Relocate Origin enabled in the DXF Options? That would shift the trace to 0,0 so may account for the error but that would be purely a shift and not an overall size change.
The other option you have is to trace the design in Inkscape (or another program) and import the DXF.
I do many jobs where I engrave the design and then use Sketch Plus (Overlay) to contour cut-out with multi-pass and the alignment is excellent. That's the reason I added the feature.