Why are Screenflow files so huge?

I downloaded Screenflow as a screen recording tool. It works fine and everything but I recorded a 24 minute episode of a TV show I had streaming on my baseline Retina and the .screenflow video that came out was over 13GB!

Why is a simple video recording of my screen so enormous? At most I thought it would be a couple hundred MB (the video itself was ~300MB).

Thanks!