You are correct.
I just had another look at the DMM catalogue.
Their smaller "Nema 34" size is in fact the size of a Nema 34.
Their larger "Nema 34" 750W size isn't. The bolt hole circle is 100mm instead of 98.4mm, and the pilot circle is 80mm, which is a bit larger than a Nema 34's. The shaft size is 14mm, but I have seen nema 34s with 12.7mm shafts, and also 13mm shafts, so not sure there is a standard there. I'm a bit annoyed that they list it as "Nema 34".
It wouldn't be a big deal to make an adapter plate and drill the hole in the pulley a bit bigger for the larger shaft. I don't know. But you are correct.
Ken,
What kind of top speed and acceleration are you trying to get with this upgrade? What would you be happy with?