Satoru Iwata, former Nintendo president and legendary video game developer and Japanese businessman, is often credited with helping Pokémon Gold and Silver contain two regions in one game.
If you're not in the know, Pokémon Gold/Silver managed to let you explore both the Kanto and new Johto region in one. Quite the feat, really. But, apparently Iwata's involvement was less to do with the size of the game, and more to do with its speed.
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According to the latest episode of Did You Know Gaming? Iwata managed to get the game running much smoother on the Game Boy. Some of the improvements are quite marginal, but overall they worked to make the games run a lot better, especially when encountering wild Pokémon.