GungHo Online's Puzzle and Dragons is currently making $850 million per year, while Supercell's Clash of Clans and Hay Day are doing similar numbers combined.
But the Japanese and Finnish developers certainly aren't acting as rivals.
Indeed, the two companies are now working together in a mutual mash-up.
Puzzle & Dragons will incorporate special Clash of Clans content including a themed dungeon and exclusive Clash of Clans monsters. The dungeon will have multiple difficulty levels for players to defeat.
Meanwhile Clash of Clans and Hay Day will feature cross-promotions for Puzzle & Dragons including videos and content in Clash of Clans' Battle Log
The collaboration will launch on 24 June and will run for two weeks.
Whistling in the wind
Of course, the irony is the three games are unlikely to have much cross-over.
Puzzle and Dragons is big mainly in Japan on iOS and Android, Clash of Clans is massive in the west on iOS-only, while Hay Day's iOS audience is western, but female so unlikely to want to play either an RPG or a strategy PVP game.
"We are very honored to work with GungHo and learn from the masters of free-to-play gaming," said Ilkka Paananen, Supercell's CEO, rather modestly.
"The more time we spend with GungHo, the more we realise how remarkably similar our two companies are. We are deeply committed to helping each other expand into new markets and equally committed to having an incredible amount of fun working together."