I wonder if anyone else noticed this: When the transit started, I thought it was amazing how big Venus looked through a scope, and unaided it was just barely discernible as a tiny dot. Then at the end of the transit (before sunset anyway) Venus looked even a good deal more huge, and unaided it was definitely easier to see and bigger without having to look so closely. I'd say it appeared 50-80% larger at the end than at the beginning.

I wonder what caused this. Venus definitely wasn't moving significantly closer to Earth during the transit. One might think it is the Moon illusion, where the Moon looks bigger when near the horizon than when it's high in the sky. But that is caused by the relative closeness of surrounding terrestrial objects, which doesn't apply. The Venus transit was different in that:

1. I was viewing it through a telescope without other objects visible beside the Sun (until it set over the mountain at the very end).

2. Even unaided, Venus's size was mainly being compared to the Sun's.

My guess is it may still have to do with the Sun being high in the sky at transit start, but low, of course, at sunset. Because Venus looked incredibly sharp of a sphere early on, but certainly lesser defined at sunset. All that added air density seen through when an object is low in the sky blurs an object, naturally. And perhaps that can have an aggregate effect to also cause the object to appear larger as well.

What do you think, does this sound likely, or have any other ideas?

As far as Venus appearing sharp early on and less defined at sunset, I agree that was due to atmosheric turbulence closer to the horizon. While that might have some affect on the apparent size differential, here's another hypothesis (I have nothing to back it up, just pure conjecture on my part). The Sun visually appears darker along its limbs compared to the center of its disk. As Venus moved closer to the center of the disk, the brighter part of the Sun may have caused Venus to 'pop out' more due to the higher contrast. That, in turn, may give the optical illusion that Venus appeared larger?

An observation that might clarify this would be to view Venus normally through a telescope, in the day, high in the sky. Then again when it's low over the horizon. If it seems bigger low, then maybe it's the atmospherics. If not, maybe it has to do with something else, like the contrast with the Sun.

However, normal viewing is with Venus bright, and the background not so much, even in the day. Whereas with the transit, Venus was totally dark, and the background across the Sun very bright. So, I wonder if this could affect the atmospheric size phenomenon, to whatever degree it may or may not be occurring.

I tried viewing Venus today, but 6 1/4 degrees is still too close to the Sun, apparently.