The Ringworld was built by Pak Protectors, fleeing the Pak Homeworld to create a home for their descendants because the Pak Homeworld is a very dangerous place, being the site of constant competition for limited resources and room to live. By building the biggest possible habitat (the Ring used essentially all of the non-star mass in the Ring's solar system) the time until the Ringworld is full, and protectors once again face a battle-royale of everyone against everyone else is maximized. If they could have made it even bigger, I suspect they would have.

Furthermore, smaller Ringworlds in the same solar system would compete for the remaining resources in the system, like solar energy or materials (to build the next small Ring), and any left-over asteroids would make great weapons to use against other small Ringworlds.

The Ringworld Engineers reveals that the Ringworld was built by Pak Protectors; by bringing in the Pak, TRE also brings in the events of Protector, which explains what happens when protectors compete for resources and land for their descendants. Per TRE, the builders of the Ring intended to build a home for their descendants. 

By building the biggest possible habitat (the Ring used essentially all of the non-star mass in the Ring's solar system) the time until the Ringworld is full, and protectors once again face a battle-royale of everyone against everyone else is maximized. If they could have made it even bigger, I suspect they would have.

Furthermore, smaller Ringworlds in the same solar system would compete for the remaining resources in the system, like solar energy or materials (to build the next small Ring), and any left-over asteroids would make great weapons to use against other small Ringworlds.

The Ringworld was built by Pak Protectors, fleeing the Pak Homeworld to create a home for their descendants because the Pak Homeworld is a very dangerous place, being the site of constant competition for limited resources and room to live. By building the biggest possible habitat (the Ring used essentially all of the non-star mass in the Ring's solar system) the time until the Ringworld is full, and protectors once again face a battle-royale of everyone against everyone else is maximized. If they could have made it even bigger, I suspect they would have.

The Ringworld was built by Pak Protectors, fleeing the Pak Homeworld to create a home for their descendants because the Pak Homeworld is a very dangerous place, being the site of constant competition for limited resources and room to live. By building the biggest possible habitat (the Ring used essentially all of the non-star mass in the Ring's solar system) the time until the Ringworld is full, and protectors once again face a battle-royale of everyone against everyone else is maximized. If they could have made it even bigger, I suspect they would have.

Furthermore, smaller Ringworlds in the same solar system would compete for the remaining resources in the system, like solar energy or materials (to build the next small Ring), and any left-over asteroids would make great weapons to use against other small Ringworlds.