LogFAQs > #963593172

It's not really that short IMO. it's slightly smaller than kanto but not by much. It takes about 80% as long to play through as kanto. Kanto is neutered in GSC so it takes about 30-40% as long to complete as it did before (unless you skip all the trainers and content)

Then you also add in the new content outside of maingame Johto, like the route connecting Johto to Kanto, and Mt Silver and the route leading to it.

if you play HeartGold/SoulSilver Kanto is much more fleshed out and has all its dungeons and forests back, plus johto is expanded

In any case it's still easily bigger than Galar.