So another odd thing I'm seeing with this VDI, it's showing the size incorrectly. It shows 180GB of 180GB used up (on thin provisioned SR, both the old and new are), however the VM is only using 140GB of that 180GB.
Something definitely went wrong with this VDI during transfer, just not sure what.
I will say that I increased the VDI size again and now it displays more accurately, showing 180GB of 185GB used (both in XOA and with vhd-util). Almost behaving as if this was at one point on a thick provisioned SR or something.
Just to avoid issues I'm maybe tempted to create a fresh VHD, copy data to that, then delete this one.