When you use the Build Preview Movies command low resolution preview movies are created from your source movies to use even when the original source media is offline.
The following compression presets are available. The approximate file size for one hour of preview footage at 160x120 resolution is shown in each case. (In the Professional Edition you can create your own customised preview settings using any available QuickTime compressor.)
The difference in time to compress between the speed optimised and normal presets can be a factor of five or more, but with a corresponding difference in the resulting file size and quality.
Note that you should not access the preview files from the preview directory directly. They are private to CatDV - this is why they have obscure names like "25,123,4210,64x.mov". Instead, use Export As Movie(s) command, where you can choose to export existing previews as either reference or self contained movies without recompressing them.
When building previews in Live Capture (using the Scan & Build Previews command) time is at a premium, so this always uses a 'fast' Motion JPEG codec, with IMA 4:1 12 kHz audio. The frame rate and JPEG quality setting are controlled by the Live Capture preview settings in Preferences.
As less time is available to compress each frame the Live Capture preview file sizes may be bigger than when building previews from movies that have been captured to disk. You may choose to use a smaller preview size for Live Capture therefore.