I have to somewhat agree with Maxharvard. Though I expect you're not using a different lens, just different focal lengths. The issue is really that the longer your focal length gets, the narrower your angle of view. How it affects a shot in full sunlight is that you're significantly cutting the light getting through the lens.
Another aspect at work here is where you're metering. Take your first two photos for example. I suspect you're shooting in aperture priority or shutter priority mode, or maybe even auto? The first at 46mm is possibly metering off of the sky or the horizon, which will naturally make the camera want to compensate by a faster shutter, resulting in a darker image. The second might be metered off the hilll, making the camera compensate by a slower shutter and a brighter shot.
So what's the lens hood really for? Glare. Especially if you have a wide-angle lens, it's purpose is to cut the amount of sunlight shooting ACROSS the lens - light that throws things off kilter. It's a noticable flare in the final shot, and I don't see it in any of these photos. It is true that it will cut some of the light getting into the lens, but not very much - especially in full sunlight, the shutter speed will easily compensate.
So what to do when shooting in a lot of full sun? Get a Circular Polarizer filter. If you need to slow down the shutter more than that - like if you want to shoot moving water so it blurs - get a neutral density filter. You should also learn to at least use the Exposure Compensaton (Exposure Bias) feature because you can't trust the meter 100% in full sun. I don't have my color corrected monitor (laptop is out for repair), but it would appear as though the first shot is under-exposed while the second is slightly over-exposed.
I know that was long winded and probably more than you asked for...but I assume you're used to my ramblings by now. ;P
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