I have only played a few games on smaller maps, and all of them were finished quickly, I admit, I never played any of these mediam map games beyond normal difficulty, but I think the AI is far more difficult to defeat on larger maps. A war is hardly ever decided quickly in larger maps where I and the AI have large empires with possibly dozens of planets. it's not uncommon for me to play a game that lasts over two weeks, although admittedly that's also because I like to drag things out a little.
If the AI looses 10 planets in the war, it is still capaple of mounting a large offensive due to other productive planets in their possesion churning out ships as fast as possible. At this time, the distance my transports have to travel is quite big, and the war becomes more about logistics (not the actual game logisticpoints) Can I replenish my fleets and armies quick enough? There comes a moment in a war when the AI will strike back, and might even take a few planets left poorly defended if I'm not careful. Sometimes it is very appealing to get that extra planet, directing a fleet there, only to find that the AI has a large fleet behind me taking back planets I thought where safe. Not to mention the sudden declaration of war from another race, which doesn't like my aggressive move on their friend, and decides to attack me on the other side of my empire, forcing me to fight a war on two fronts. Great fun!
There are a lot of factors involved I did not find in smaller maps, where wars tend to be short, with each side only having a handful of planets. If anything, I wish I could play on even larger maps beyond gigantic, but I doubt my PC could handle it.
On a side note: I recently began playing on the Metaverse, It does certainly add something to the whole game experience.