They are both compressed.. The new sat HD channels are in mpeg4, while OTA is still in mpeg2 (same as the channels in the 70's)
Mpeg4 takes less room than mpg2..
All very true. Might I add that there is more uniformity between sat channels than there is between OTA channels. Some OTA broadcasters allot up to 18 Mb/s for their HD channel. Others use far less, especially if they are multicasting. FOX uses stat-mux so their level normally swings between 9-13, hovering at about an average of 11, with a quality that is probably equivalent to non-statmuxed signals at 13 or 13.5 Mb/s.
Ironically, that means that a prime time HD show on FOX (averaging 11 Mb/s) can take up far less room than a syndicated show in SD on the very same station, since the station itself may allot more space to their local broadcasts than FOX does to the network feed.
But as houskamp says, the DTV versions of local OTA HD are converted to MPEG-4 which is about 30% more efficient, so that version will take less space on the HDD than the actual OTA signal. Since in my market there seems to be no discernable difference, I opt for the MPEG-4 feeds unless I am expecting rain fade (which OTA sails through much better).