I have done some pocket engraves and inlay, but not since evansd2 convinced me to cut all the way through for inlays.
With my pocket engraves I have encountered similar issues. One issue, not the one you seem to be having here, is when accounting for kerf a filled object doesn’t seem to be the same size as the outline of the object. Maybe this is just an Inkscape thing, but it took many attempts and debugging sessions to figure out a 1x1 square cut is larger than a 1x1 square engraved*. I bring this up, because if you start messing with the size of the object things can start to go weird quickly.
What I believe you may be running into is that the material removed by the laser in the middle of an engrave is not the same as at the edge. If you engrave from point 0 through point 10, at points 1 through 9 the laser is blasting away point n and some of points n-1 and n+1. At point 0, point -1 is getting a little heat and at point 10, point 11 is getting a little heat. These (-1 and 11) are the points that define the boundary of your pocket. You decreased power from 100 to 80, so theoretically your pocket should have shrank when it grew. While the proofgrade settings for walnut and maple are the same, I have found walnut to burn a bit easier. So it is possible you have a larger pocket because more walnut was burned away at the edge, even with the lower power setting. Or it is because pocket inlay is even more frustrating than cut through inlay.
*Yes, I have geometric bounding boxes selected.