You can find that in the phone storage settings:
iOS: 12 G
Keynote: 498 M
Numbers: 482 M
Pages: 455 M
Clips: 213 M
Maps: 81 M
Watch: 70 M
Find My: 60 M
Music: 38 M
iTunes U: 35 M
Support: 34 M
Podcasts: 32 M
Books: 31 M
iCloud Drive: 30 M
Freeform: 19 M
Fitness: 18 M
Notes: 17 M
Journal: 15 M
Home: 10 M
App Store: 8 M
Weather: 8 M
Mail: 7 M
Files: 4 M
Health: 3 M
Measure: 3 M
Voice Memos: 3 M
Calendar: 2 M
Clock: 2 M
Safari: 2 M
Shortcuts: 2 M
Translate: 2 M
TV: 2 M
Calculator: 1 M
Facetime: 1 M
iTunes Store: 1 M
Tips: 1 M
Wallet: 934 K
Messages: 860 K
Photos: 791 K
Compass: 712 K
Camera: 635 K
Contacts: 598 K
Phone: 570 K
Magnifier: 516 K
Passwords: 213 K
There's also an "Apple Inc." listing, which appears to be "shared" between a lot of their apps which clocks in at 204MMy takeaway from having gone through the list and compared to the various 3rd party apps:
1) Apps can absolutely be smaller. Plenty of stuff in the <200MB range including things like Signal, OBD Fusion and Infuse
2) Games are often big, but there's a surprising number of "simple" apps that are larger than some of the games
3) The largest apps seem to be from companies that you would expect to be doing the most tracking of your data
4) Apple's first party app sizes probably explain a little about why they weren't in a hurry to upgrade storage sizes