petrockblog
Keymaster
Post count: 1827

It is clearly detailed in the setup instructions on the wiki

https://github.com/retropie/RetroPie-Setup/wiki/First-Installation-%28RetroPie-SD-Image-2.6%29

The image is kept as small as possible – if we had it all partitioned for say a 16gb card, you would need to write 16gb to get up and running. We also don’t expand on first boot, as it’s possible users may want to customise their partition setup. It’s a simple process to expand, and Raspbian works in the same way.