Apple's Quicktime is the default media player that comes with Mac OS X, with the added feature of allowing you to resize video files. While you can't specify custom sizes for your videos in Quicktime -- even with the Pro version -- you can make them smaller by optimizing them for the Web or an Apple device. Quicktime lets you set quality levels as well, which also affects file size.
Open your "Applications" folder and double-click the Quicktime icon.
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Select "File" from the top menu bar and then select "Open." Alternatively you can press the "Command" and "O" keys at the same time to open a file. Locate the video you want to make smaller on your Mac and click it to select.
Select "File" from the top menu bar and then select "Export" for web.
Click the box next to the desired size. Quicktime lets you export for the iPhone at 56kbps, which is the smallest and 900kbps, and for your Desktop at 1.5Mbps. Keep in mind that the smaller you make your file, the more you will notice a loss in quality.
Click the "Export" button to export your file. The export process may take several minutes or longer depending on the size of your original file.