It definitely isn't unless yo are basing it on shrinking the graphics. The arcade is 4:3, C64 monitors are 4:3 so the game can be scaled to fit the whole 4:3 area without the score on the right...with the exception of that light blue border area on the C64, but it can fill the Darker blue area completely. The NES, Master System, and others do not have this score area on the right..the gfx fill the entire screen.
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I am sorry but clearly you don’t know what you are talking about. If you compared the actual resolution, that means the number of pixels, not what you see on the naked eye, of the arcade, the C64 and the console version of Bubble Bobble, you would see that they are all already basically identical. Go check it out on wikipedia or any other technical site if you do not believe me. So no, despite the fact that apparently they do not fill the screen, the graphics on C64 are actually the same size as the arcade. There is a sidebar because the C64 screen actually has a higher horizontal resolution, so more pixels than the arcade or consoles. On the other hand vertical resolution is lower.