Isn't it odd that most people only have one (or very few) kinds of city in mind when considering this question. I have always found cities to feel very large, large, or small - all for a wide range of experiences/qualities. And, very roughly, they tend to line up according to population.
Even if we limit it to physical built structure, LA feels very big, as does Houston, but also the obvious London, Paris, New York. Phoenix can feel massive when driving around it's endless expanse until you see the core skyline. The reaction is 'wow, that's barely bigger than Regina's.' Tiny city core, but Phoenix still feels large.
In other terms, crowds feel like a big city quality. LA has limited crowds, save for in cars on freeways. It still feels very big. Vancouver has become shockingly crowded with people in the central areas (Lonsdale-Dtn-Broadway). When I returned after a year plus in Vienna/Zurich, where crowds are huge in some parts, Vancouver didn't seem less crowded, just differently crowded, more spread out. Vancouver streets have people all over in the core, even past midnight there are people out, along the parks, seawall, west end, Yaletown, and so on. There are a lot of chains but there are tons of unique restaurants, bars, galleries, Dance Centre, Cinemateque, Film Society Cinema, Van Film School, two universities, two stadia, Van Playhouse, subway stations, Cruise terminal&ships, seaplane terminal, heliport, high end fashion retail zone, hipster gastown, design and furniture shops of Yaletown, Churches, VAG, 3 community centres, Orpheum, Commodore Ballroom, and a hunderd thousand residents, and on I could go. To say its empty and closed after ten is ridiculous. How could I compare it to cities like Vienna (similar populations)? Vienna: fewer good restaurants, more cultural features, different architecture, bigger? No, different. And contrary to the stereotype, even within the ring, Vienna can have huge crowds on the core pedestrian malls, but turn two corners and you can sit and have a coffee or meal at a shabby little outlet beside a train overpass in a vacant little rundown square: an awesome quality. Bigger? In some ways, in others smaller. Overall? Just different - both feel big.
There are dozens of metrics beyond these. Bigness, for me, varies across numerous of them for each different city. City vs City analysis seems fruitless to me. Paris vs London, vs Istanbul: who's the biggest? In what? Overall? Who cares. Any city over 1 million feels big. Vancouver size feels bigger, Montreal size: a bit bigger, Toronto size: very big, LA size: bigger again, New York, bigger again, Tokyo: flippen huge. And all for a variety of different reasons.