Saint Peter's Basilica was built, in part, to overwhelm, and that presents any painter attempting to capture it with a genuine problem: how do you make a canvas convey a scale specifically designed to exceed ordinary human comprehension? Panini's solution was populating his view with crowds of small figures, clergy, pilgrims and visitors scattered through the nave, each one rendered at a size that, by contrast, makes the surrounding architecture read as vast. Without those tiny bodies for scale, the basilica's columns and vaulting would simply look like an abstract pattern of stone; with them, a viewer's eye does the arithmetic automatically and understands, correctly, that this building dwarfs everyone inside it.
That compositional trick reflects something true about the basilica's own original design intention. Saint Peter's interior was built over more than a century, involving Bramante, Michelangelo, Maderno and Bernini among its successive architects, each contributing to a structure meant to assert the authority and permanence of the Roman Catholic Church through architecture that a visitor's body could feel as much as see, the sheer distance to the far end of the nave and the height of the dome registering almost physically rather than only visually.
Panini's crowded, carefully populated interiors are, in that sense, doing exactly what the architecture itself was built to do to a visitor standing inside it: making a single human body, and by extension a single human viewer looking at the painting afterward, feel appropriately small against something built to represent an institution far larger and more permanent than any one person passing through it.