You probably won't stumble onto this so here is a thread about rebuilding that shock:
There is a plastic topping-spacer in that shock that limits the travel to 7.5 inches. If you remove it, the shock extends to allow 9" of travel (same as the fork).
This is the post:
You can see that the bike doesn't really lean over much more than stock. I think Cagiva intended for it to work that way. It allowed those 'in the know' to modify the shock and get the full offroad capability out of the chassis.