3GB isn't crazy at all for 22 seconds of uncompressed HD video.
Depending on what you want to do with the footage in the long run, you may want to render out a lossless intermediate render (something like a Quicktime movie using PNG compression), and then compress that movie using Adobe Media Encoder (or a similar compression tool) to a delivery format more suited to your file size concerns (i.e. 2-pass VBR h.264 encoding with a substantial bitrate). If you don't have the storage, then you can always ditch the uncompressed/losslessly compressed render after you have your final product. The reason you'd want the intermediate stage is so you can obtain the best compression possible. XDCAM (whichever flavor you're using) is a highly compressed and limiting acquisition codec. You don't want to render back to it, most likely.
So, the question is, what are you going to do with your final product? Web?
Hi - Thank you so much for your help - its all starting to make a bit more sense.
I have produced videos before and uploaded them onto the interweb and to do this I used h2.64 and it works perfect
I and now thanks to you I understand uncompressed files
My goal is to make a 12min short film and I want to have the project available at its highest quality in case anyone actually likes it and would like to distribute it.
So - when I create an asset in after effects for my short I will create two versions one h2.64 for quick viewing and editing and then replace this with the full quality AVI when I finished editing
those this sound ok