Ok, round two. Attacked the same planet as before, this time with 584 cruisers and 134 battlecruisers. This time the planet no longer had any orbitals or anything, so it was a straight up fight. I have already taken a different home world with the same population, using less ships. This time, all my ships destroyed, planet remains in the other player's control.

Why did my bigger fleet not manage to take the planet?