Yea, the Jetbikes used to be mounted on the smaller bases, but that changed over the years.
The reason you see the smaller bases in some pictures is because they are taking pics of studio models that are often almost, or over, a decade old. in this case I am speaking of the Sam Hainn jetbikes you see in WD and on the GW website. The pic they put on the old Windrider box set was the best telling point. It showed newer and older jetbikes in the same pic. The older ones had small bases, while the newer bikes had the larger bases.