sorry, I just re-did the test using the svg file from comment 1. It appears that part of the problem is due to the difference between visual bbox and geometric bbox. The rectangle in the svg file from comment 1 has a dimension of 99.458 mm not 100. The visual bbox is 100 but the geometric bbox is 99.458 and this is what controls the size of the dxf file. If I export this to dxf I get a dxf size of 99.458 as well, so the result is as expected.
sorry, I just re-did the test using the svg file from comment 1. It appears that part of the problem is due to the difference between visual bbox and geometric bbox. The rectangle in the svg file from comment 1 has a dimension of 99.458 mm not 100. The visual bbox is 100 but the geometric bbox is 99.458 and this is what controls the size of the dxf file. If I export this to dxf I get a dxf size of 99.458 as well, so the result is as expected.