I have both. A quick butchers without getting the calipers out would suggest that the cases are the same size. The only difference *might* be in the caseback that looks to be slightly deeper on the Broad Arrow. But it's pretty close in size, if there's any difference at all. As you've found out, Speedies, despite the 42mm size, sit well on smaller wrists, especially on straps.
If you want, I'll try and get some pictures up, but it won't be today. Garden duty calls.....