Here are my thoughts on stabilization, starting with some basics:
1. A larger sensor, for given focal length and distance, makes mechanical (optical) stabilization more challenging.
2. The challenge increases with focal length and with close-ups.
At the same telescopic focal length and at close distances, the optical stabilization of the TM900, with its relatively tiny sensors, is superior to that of the GH3 power OIS and the AX100 in active mode. In active mode, the AX100 is giving you over 500mm focal length at the long end. Neither the GH3 lenses I have go that far and the TM900 does not either, yet I can get reasonably stabilized shots (baseball game) at that extreme end with the AX100. At modest focal lengths - say,wide to 60mm, there is not much difference at normal distances.
The AX100, as has been observed, is a big handheld camcorder. Opened up, it is almost all lens - to get a 12X zoom lens with a 1" sensor and a reasonable aperture (f2.8 at the wide end) requires volume. To improve on stabilization would also require even more size and weight (compare the power OIS Panasonic lenses with their non-stabilized counterparts).
Speculation: The stabilization prowess of the AX100 is a compromise, to retain size, just as the 60Mbps in 4k mode is a compromise, to save on energy (heat) in a compact body and to enable the use of regular sd cards. Active mode helps a lot (without Active mode, you cannot handhold telephoto shots) without any obvious decrease in quality.
What was not compromised was the resolution prowess (same processor as the pro model).