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.
Two of the key structural features in Roman architecture were the arch and the dome, both of which were more easily constructed from concrete and/or brick than from marble.
Additionally, concrete and brick were more easily and cheaply manufactured than marble. Concrete and brick can be made from cheap and plentiful ingredients by relatively unskilled workers, whereas the quarrying and cutting of marble is a more expensive process and requires workers with greater skills.