In many games I play do I think that the game is decided too much by who grabbed the most planets in the initial fase and not by the actual management of their empires after that.

My first question is... What level are you playing?

I have played a few games at the higher levels and it can be very difficult to out colonize the AI. At the mid range level, I have been out colonized on several of my games but managed to overcome this disadvantage fairly easily, usually through tech means, but I have resorted to military conquest as well.

I believe it is not the amount of planets you colonize, but the quality of said planets. During my initial strikes against an enemy, I take out their best planets first, as any good general would do. Destroy their ability to wage a war and victory will be yours in short order.

However, I do believe there are a few custom map games and mods out there that start in the way you are suggesting. Maybe you should look into them.

Good luck to you sir.