This is actually quite valid for the AI to do, and they'll do it frequently on both platforms -- game logic is literally identical code between platforms, by the way, so this sort of thing really can't vary (otherwise there would be major desyncs anytime people played multiplayer between platforms).
To your actual core note, the AI ships are stalking you -- that's a new behavior that was added sometime in the 4.0 to 5.0 time period. Rather than just impaling themselves on your fleet, they'll hang out until they sense you are vulnerable, then go in for the kill. If you want to kill them first, you'll have to go after them in their own territory. The big ships aren't blocking anything, they are probably just having trouble finding a place to settle down since there are so many smaller ships.
On the PC side, you might have had a lower difficulty setting (which affects AI behavior), or the planet in question might simply not have been as well defended, or something of that nature. That's all I can really think of; but this should be happening on both platforms if your planet is sufficiently well defended and the difficulty is sufficiently high.
Hope that helps!