There are lots of reasons depending on many things why it might happen.
One reason for audio desync I've just recently learned is differences in the sample rate of the audio set to be captured/recorded and what the sample rate is set to record with the audio driver. (not sure if this applies to power director or not, but some software it does)
Over a long period of time, capturing at a non-standard frame rate can cause audio desync (like 30fps instead of 29.97 or 60fps instead of 59.94)
About large file sizes, the less the video is compressed, the less processing power is needed for the saved video data, which in turn may improve capture performance.