The term "full-power" was used throughout the the original 2000 Report and Order that established LPFM (15 FCC Rcd 2205) (likely to contrast from "low-power" stations). The intention is to prevent LPFM stations from being used as translators. This came up in the discussion of local originated programming.
(R&O at 170) Commenters generally agree that LFPM stations should not be used as translators for retransmitting full-power station programming. The Civil Rights Organizations and Gary L. Nixon argue that an LPFM operator should not be permitted to operate as a translator or booster, except where an LPFM station might retransmit another station’s programming for the purpose of student training. Nixon also notes that LPFM stations should be prohibited from using any satellite programming. UCC, et al. states that the Commission should not allow low power stations to replicate another station’s programming because it would turn the purpose of low power radio, to provide local programming, on its head.
(R&O at 172) We do, however, agree with commenters that LPFM stations should not be used for retransmitting, either terrestrially or via satellite, the programming of full-power stations. This would significantly undercut a fundamental basis for the establishment of this service. This prohibition against LPFM stations operating as translators also promotes locally originated programming by eliminating a significant avenue for obtaining non-locally originated programming.
(Resulting codified rule §73.879) An LPFM licensee may not retransmit, either terrestrially or via satellite, the signal of a full-power radio broadcast station.
It has nothing to do with amount of power. It has to do with using the service for a purpose it was not intended for (a 24x7 translator of a full-service station). Of course, EWTN, 3ABN and NTD get around this by not having a broadcast station to rebroadcast but instead just rebroadcasting a satellite television/radio network.
Keep in mind too that this was written at a time when organizations were allowed to own up to 10 LPFM stations and the landscape was a little different.
Therefore, it could be assumed that "full-power" stations can include full-service FM broadcast stations (Class A, C3, B1, C2, B, C1, C0, C), all AM broadcast stations and we can even stretch to include full-service television stations. The interpretation could also take into consideration Class D FM stations, Class A and LPTV stations as these outlets can also originate their own programming.