I have used the IR Defense 640 ~1.25x Patrol and the 2.5x 35mm MK II, which gets right to the native magnification issue and to a lesser extend the lens size difference.
IMO, about 2-3x native magnification is the best compromise for night hunting.
Bigger lens are better lens, but I wouldn't sweat any difference if the unit was otherwise what I was looking for. You will loose some performance relative to a bigger lens in crappy thermal conditions, and that is especially true with background. But critters still stand out, even if range is reduced.
JPK