Check out or any book on view camera technique or macro photography for different approaches to computing bellows factor or magnification factor. There's a thread here on bellows factor, where I think I posted my magnification/exposure table as an MSWord DOC file.

With small and medium format cameras, it tends to be a concern only in the macro range, and with 35mm it gets taken care of typically with TTL metering, since an in-camera meter is looking through the lens, and the effect of magnification factor on the film is the same as it is on the meter.

When you're shooting large format, many subjects that you might not have thought of as "macro" with 35mm are suddenly in the macro range. A tight portrait on 8x10" can be at a magnification ratio of 1:3 (size of the film:field of view at the subject position), and the same on 4x5" would be 1:6, requiring additional exposure. With that puny 35mm camera, the same portrait would be 1:24, requiring no exposure compensation at all. If you were photographing something small like a spider with the puny format camera, though, you might be at 1:2, and if you were lighting it with flash and didn't have TTL flash metering, you would have to compute exposure factor there too!

With landscapes, though, the subject tends to be far away, so bellows factor isn't relevant, which is why your other exposures were okay.