The Forerunners built planet busting ships in [i]minutes,[/i] and could make Halo rings in a matter of weeks. Their fleets were in the high millions as well. And if we're going to do space combat, let's not forget the Onyx-Class Sentinels. Just 40 of these could annihilate a Covenant CCS Battle Cruiser; imagine what millions of them could do to an Imperial fleet. And on the Precursors: [quote]The Precursors based their existence around the philosophical concepts of the Mantle, Living Time and a meta-technological mechanism known as neural physics, which enabled them to manipulate the fabric of the universe. According to the philosophical aspect of neural physics, the universe itself was a living entity, though vastly different in nature and scale from organic beings.[/quote] They don't use space magic, they bend and rewrite the very [i]fabric[/i] of the universe. Their constructs are nigh-indestructible, even to Force users.