Flash higher CPU usage under OS X than Ubuntu?
Since you can't make any assumptions about how Flash for Mac or either Flash for Linux are implemented or how they utilize efficient APIs of their host OS you can't make any statements about why performance behaves so differently.

As long as graphics acceleration for video playback is available on both OS's it should be Flash's task to correctly use it.
You might want to check if Hardware Acceleration is enabled in Flash on Mac OS. To do this, right-click into a Flash-Applet and select "Settings...". Then, in the first tab (Display), you should be offered a Check-Button labeled "Enable Hardware Acceleration".



Performance is also dependent on the video codec of the media file. HD content from youtube for example only takes 10% CPU-time for me, whereas Flash content from other websites may also get my fans started.