Thanks Kitkun
Well, now that I know to expect a max, I'll do a better job at fleet composition. Also, I got an unlucky random map, with no choke points - so I had to send smaller fleets all over the place.
Discovered one trick though. When I withdrew a large fleet back 1 planet away from a frontier planet, the AI attacked with a smaller fleet (or maybe a less powerful one) and I was able to move back in and wipe them out. Had I done this a few times (and the AI didn't "learn"), could have eaten a big hole in the AI fleets.
Live & learn....