There are several factors that contribute to volume being lower on HD channels than it is on SD channels.
The default sound format for HD channels is DOLBY 5.1 sound. This requires us to balance the sound on six different audio channels, which lowers the overall volume slightly. In addition, the six different audio channels are tuned to a higher standard than stereo sound which also lowers the volume on HD channels.
While these factors may reduce the overall volume, the balancing and tuning result in clearer more vibrant sound with less distortion. However, it may require you to turn the volume up on HD channels.