The romans created larger interior spaces in architecture than the greeks because:

One way that the Romans were able to create larger interior spaces in their architecture than the Greeks is by the use of concrete, which is lighter, stronger, and less expensive than marble. The Romans also became very skilled at the construction of bricks, which are also stronger and less expensive than marble.

Because concrete is lighter than marble, concrete buildings do not require as much interior support (e.g., columns) to hold them up. Fewer columns or interior support structures mean more interior space.

I got this information from: https://www.enotes.com/homework-help/why-were-romans-able-create-larger-interior-spaces-445813

I hope I helped:)