Something I still don't get..
How and why did the Federation happen?
Did we know about aliens before the Bugs?
Why was there a need for such a huge space force?
In the book there are other alien species that they are also at war with. I think the fleet in the movies can be explained the same way our real militaries can be though. They're pretty much stationed around the galaxy at whatever planets have military installations, like real life militaries having bases in other countries. They all just formed up when the war started. I assume all that because the Rodger Young was just flying on its own before the attack on Earth so it would be like a battleship just patrolling an ocean area. If humanity expanded as much as they did into space in this universe then it makes sense to have military on each planet, and that'd have to be a ton of ships ready in case anything happens, alien or human.
So basically: humanity is spread out across tons of planets which means they have tons of military installations.