Firstly apologies for the high anorak rating of this post, but it concerns an issue that has been puzzling me for a few years.
I have often noticed that the view through porro-prism binoculars looks smaller than that through roof-prism binoculars of the same nominal magnification. I have tried several makes of binocular and always seen the same effect. I thought that I was going a bit potty, but others have said the same. Anyway, today I compared the view through one optical assembly of an 8x porro prism glass with that through one optical assembly of an 8x roof prism glass. Each had almost the same field of view. Well each image appeared to be of the same size i.e. the psychological impression was one of equality of size. Then I tried looking through each instrument normally. Lo and behold the size difference re-appeared! This indicates that the illusion is nothing to do with the nature of the prisms. Oddly when considering one glass in isolation, the image through one optical assembly seemed bigger than the image seen through both!
I suspect that this is a psychological illusion created by the brain when presented with depth information. Porro-prism glasses usually have a much greater seperation between the two objectives, and hence a greater 3-D feel to the image. It looks to me as if the greater 3-D feel has the side effect of tricking the brain into 'seeing' a smaller image.
Anyway, if this idea is true, then a corollary is that the image through an inverted porro prism should seem smaller than that through a roof prism binocular, assuming for the sake of argument the same field of view and DOF.
Did you try that with both eyes in turn Lief? Maybe your right eye just sees things differently than your left ;-)
But seriously, I noticed something interesting also, which I can't verify at the moment because it is quite dark outside now. I found that porros present a smaller image close up, but with a distant object the size seems the same as the roofs. That would reinforce Lief's hypothesis, as the lens spacing would have less significance with distant objects. I'll have to check again in the morning.