Some older Macs have video cards which do have not enough Video RAM to handle larger dimension projects. On those computers, a large capture or export may be cropped. Affected computers could have a GMA 950, GMA X3100, or Radeon X1600 video card. These Macs were mostly released in the 2006-2007 timeframe.
Workarounds would be to keep your recordings and canvas size smaller (generally below about 1024 pixels), use the internal display rather than a larger external monitor, or use a newer computer with a more capable video card.
Our developers are aware of this issue and may fix it in a future release.