The shifters were not all that much smaller than my cyborgs -- they had about 15 planets when I started the invasion. They were the number 3 power in the galaxy, based on council votes. Once I finish annexing their territory, I should easily be number one. With my recent atmo terraforming and soil enrichment of all my hostile planets, I may even have a veto block. I have started researching terraforming+50, which will be a nice upgrade from my current +20. Applied to 26 worlds plus new conquests (I am not sure what level of terraforming the shifters have), I should be pretty close to half the galactic population. 
I have no idea what techs the shifters pulled from those artifact worlds. But they did expand very rapidly early; maybe some propulsion tech? Or something economic?

I have no idea what techs the shifters pulled from those artifact worlds. But they did expand very rapidly early; maybe some propulsion tech? Or something economic?

Maybe I should have tried a more focused effort, and concentrated my fleet and transports on just a couple worlds at a time. The mass invasion of many systems at once has worked, but it is a lot to manage.