It's more than just the size.
The PSU connections are likely proprietary.
The front panel connection to the motherboard are likely proprietary.
Fan connections to that BTX board are likely lacking.
Those old HP BTX things were never designed to be upgraded.
As a side not i do have a PSU bridge that would allow me to run the btx power supply with an aftermarket atx. Im just wondering if the motherboard would fit the case and line up with the rear i/o shield. as far as rigging fans and such i can get a secondary psu dedicated to the fans (and possibly a bigger graphics card) no problem