Use 'populous' to emphasize the number of people in an area, not the size of the area itself. Despite its small size, Monaco is an extremely populous urban state.
Pro Tip 2/3
Not Crowdedness
'Populous' describes quantity of inhabitants, not how crowded an area feels. A metropolis can be populous but not feel overcrowded with ample infrastructure.
Pro Tip 3/3
Neutral Tone
The word 'populous' is neutral and does not imply whether a high population is good or bad. California is more populous than any other U.S. state, with diverse economies and cultures.