This is most likely because the Mac includes both PPC and x86 versions of the binaries. Other possibilities are that the Mac version includes codecs which aren't licensed under Windows, or that the installer updates other system frameworks to make QuickTime work better. Also, the QTKit Cocoa libraries are only available on the Mac, but other than that functionality is generally the same on both platforms.
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Mac/Windows
@Size difference
This is most likely because the Mac includes both PPC and x86 versions of the binaries. Other possibilities are that the Mac version includes codecs which aren't licensed under Windows, or that the installer updates other system frameworks to make QuickTime work better. Also, the QTKit Cocoa libraries are only available on the Mac, but other than that functionality is generally the same on both platforms.
@Seperate from iTunes yet?
Since iTunes depends on QuickTime to work... no.