ANYTHING you record OTA is ALWAYS in MPEG2, as that is the OTA standard. And keep in mind, generally in many cases, it is NOT compressed as much as it is thru the dish.
OTA SD is about the same space as SD D* signals, but yes, OTA HD definitely does take up more space than HD D* signals.
And Ice, even though I understand you do NOT have HD access on your account & can't see them in HD, the DVR STILL records those OTA signals EXACTLY as they come in. So if an OTA signal originate in HD, it records in HD - you just are stuck watching it in SD!

This also applies for ALL HD DVR's being used in MPEG4 local markets & those subs that do NOT have HD access enabled - in these cases, this ALSO applies for even the HD locals thru the dish, as D* does NOT send up duplicate SD locals in Ka.