Super Mario Galaxy had 241 stars, so I don't see the big deal in his comment.
Galaxy only had 121 actual stars that you could collect again as Luigi. Even though Luigi played differently, the stages themselves were all still the same. The way Miyamoto's comment is worded, it sounds like he's saying the game will have over 240 unique stars this time, instead of just collecting the same 121 as a different character.
I do realize that saying the game will be literally twice the size of the first Galaxy was maybe pushing it, but if there are 240 unique stars, it'll still end up being a much bigger game. The previous 3D Mario's had 120 stars and yet each one of them was filled with quite a few filler stars to find. Even if over 100 of Galaxy 2's stars are easy to find filler, the rest of the game would still be much larger game.
It's like if someone took over 20 hours to find all 120 stars for Mario in Galaxy 1, we could be looking at it taking over 30 hours now to find all 240 stars for Mario in Galaxy 2. Of course I kind of wonder if Luigi would even be playable then since if you have to collect all of his stars in order to unlock the last galaxy for both characters, then we'd be looking at the game taking over 50 hours to 100%. Considering how even the RPG spinoffs don't take that long to complete, I don't see Nintendo making any Mario game that's over 50 hours long.
This is why I'm starting to think now that the most likely situation is Mario only has 120 stars to collect in his game, and then you unlock Luigi like the first Galaxy, but this time, Luigi's game is unique. This way Miyamoto was combining Mario and Luigi's modes together but since Luigi's stages are truly different this time, this is why Miyamoto sounded like he was saying the game will have 240 unique levels. This way Galaxy 2 might actually be similar in length to the first Galaxy then, but in actual content it could end up being twice as big. In which case, I'm still going to say the game is going to be so good, Nintendo could easily call it Mario Galaxy 2+3.

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