Seriously, I'm not sure what to suggest. How are you checking the size? I find the printed PDF to be inaccurate. The MS page shows a 49mm wrist height for "large", but it's unclear if that's min/average/max.
Did you try just using a Velcro extension? Put hook Velcro on post side and loop Velcro on the back of lower band. The more destructive solution would be to cut the band just back of the post side. I don't believe there are any electronics there but don't hold me responsible. You could put an expansion piece in between and reconnect.