That very much matches the average: I divided Seoul's superficy (605.25 km²) and total of "dongs" (522) into 25 (the number of "gu" or districts), and got averages of 24.2 km² and 20.8 dong.
But beyond these 19 "administrative" dong (행정동) lie 87 "court" dong (법정동), most dating from the early Joseon dynasty. Many "dong" names are engraved in national history and literature. Some designate functions, because of the kind of offices or services you would find in the area.
Most dongs are simply microscopic: taking out Pyeongchang-dong (8.92 km²) leaves an average 0.17 km² for its 86 peers, and a significant portion of what's left is covered by vast mountains, parks, or royal palaces.
Regroupments, renamings, and even extensions were performed all along the XXth century, most notably:
- on April 1, 1914: under the Japanese Occupation, redistribution and renaming of the city's 186 dongs, implementation of Japanese namings. This completed the 1910 degradation of the Capital as "Gyeongseong-bu", an almost anomymous section of Gyeonggi-do, part of the annihilation of all references to Korea's identity as an independant power. Note that when Gyeongseong was renamed Seoul on August 15th, 1948, it gained a status of "Special City" which somehow marked, beyond the independence from Gyeonggi-do, the independence from colonial rule (the final "Seoul Teukbyeolsi" label was awarded exactly one year later). Jongno-gu heads the other 6 districts : Dongdaemun-gu, Jung-gu, Seodaemun-gu, Seongdong-gu, Yeongdeungpo-gu, and Yongsan-gu.
- on October 1, 1946: many renamings, mostly denipponification reversing changes made during the Occupation.
- in 1949: Seongbuk-dong, now a diplomats' lair on the other side of the Samcheong Tunnel but traditionally a Beverly Hills for Chaebol owners, leaves Jongno-gu for newly formed Seongbuk-gu
- on October 1, 1975: new city regulations ruled on September 23, 1975 extended Jongno-gu's jurisdictions well beyond the old intra muros city, most notably :
. to the West: Muak-dong and Gyonam-dong added from Seodaemun-gu. Gyonam is on the "wrong" side of the old Dongnimmun gate and the fortress wall, but after the completion of Sajik tunnel, it obviously completed a newly formed large block around Gyeonghuigung. Incorporating Muak-dong adds the other side of Inwangsan.
. to the East: Sungin-dong and Changsin-dong taken from Dongdaemun-gu, completing the "sadaemun" tableau (the gu now encompasses the 4 main gates)
. to the North: Buam-dong and Pyeongchang-dong join the party, wrapping Bugaksan in the gu, and putting Cheonghwadae and Gyeongbokgung, symbols of yesterday's and today's power, in the center of the map.
Buam-dong (부암동 - 3 court dongs): Buam-dong, Hongji-dong, Sinyeong-dong.
Changsin 1-dong (창신1동 - 1/3): Changsin-dong (covers 3 administrative dongs).
Changsin 2-dong (창신2동 - 1/3): Changsin-dong (covers 3 administrative dongs).
Changsin 3-dong (창신3동 - 1/3): Changsin-dong (covers 3 administrative dongs).
Cheongun-dong (청운동 - 3): Cheongun-dong, Gungjeong-dong, Singyo-dong.
Gahoe-dong (가회동 - 4): Gahoe-dong, Jae-dong, Gye-dong, Wonseo-dong.
Gyonam-dong (교남동 - 6): Gyobuk-dong, Gyonam-dong, Haengchon-dong (Hengchon-dong), Hongpa-dong, Pyeong-dong, Songwol-dong.
Hyehwa-dong (혜화동 - 4): Hyehwa-dong, Myeongnyun-dong 1-ga, Myeongnyun-dong 2-ga, Myeongnyun-dong 4-ga.
Hyoja-dong (효자동 - 6): Changseong-dong, Hyoja-dong, Nuha-dong, Nusang-dong, Ogin-dong, Tongin-dong.
Ihwa-dong (이화동 - 3): Dongsung-dong, Ihwa-dong, Yeongeon-dong.
Jongno 188.8.131.52 ga-dong (종로184.108.40.206가동 - 28): Bongik-dong, Cheongjin-dong, Doneui-dong, Gongpyeong-dong, Gwancheol-dong, Gwanhoon-dong, Gwansu-dong, Gwonnong-dong, Gyeongun-dong, Gyeonji-dong, Hoonjeong-dong (Hunjeong-dong), Ikseon-dong, Ineui-dong, Insa-dong (Insadong), Jangsa-dong, Jongno 1-ga, Jongno 2-ga, Jongno 3-ga, Jongno 4-ga, Junghak-dong, Myo-dong, Nagwon-dong, Seolin-dong, Susong-dong, Waryong-dong, Wonnam-dong, Wooni-dong, Yoeji-dong.
Jongno 5.6 ga-dong (종로5.6가동 - 5): Chungsin-dong, Hyojoe-dong, Jongno 5-ga, Jongno 6-ga, Yeonji-dong.
Muak-dong (무악동 - 1): Muak-dong.
Myeongnyun 3-ga-dong (명륜3가동 - 1): Myeongnyun-dong 3-ga.
Pyeongchang-dong (평창동 - 2): Gugi-dong, Pyeongchang-dong.
Sajik-dong (사직동 - 12): Chebu-dong, Dangju-dong, Doryeom-dong, Jeokseon-dong, Naeja-dong, Naesu-dong, Pilun-dong, Sajik-dong, Sejongno-dong (세종로동), Shinmunro 1-ga, Shinmunro 2-ga, Tongeui-dong (Tongui-dong).
Samcheong-dong (삼청동 - 7): Anguk-dong, Hwa-dong, Palpan-dong, Sagan-dong, Samcheong-dong, Sogyeok-dong, Songhyeon-dong.
Sungin 1-dong (숭인1동 - 1/2): Sungin-dong (covers 2 administrative dongs).
Sungin 2-dong (숭인2동 - 1/2): Sungin-dong (covers 2 administrative dongs).
Jongno-gu keeps changing very quickly, and it's getting hard not to bump into areas under redevelopment. Population should increase significantly in the years to come. There are only 180,000 inhabitants nowadays, but registered population tops 1.4 M, and 2M people live there during daytime (many office headquarters). Some dongs may disappear (ie Pyeong-dong in the future Gyonam New Town / Dongnimmun New Town).