Yes, the size of the .gnd file should give a clue. So I ran a little test myself. In TransDEM I loaded a map and created flat DEM placeholder data, then displayed the baseboard grid in TransDEM. Drew an export mask that enclosed exactly two baseboards. Drew a line across one of the two baseboards. Made three exports for the 5m grid: "None", "Route only" and "All". This resulted in the following .gnd file sizes:
- None: 54KB
- Route only: 101 KB
- All: 148 KB
All as expected.
(Using TransDEM 2.6.3 here, but that's minor detail. It won't alter anything regarding the 5m filter. You can download and install the patch to 2.6.3 from my website.)