Could it also be somewhat similar to the "Moon Illusion" where the moon
"appears" to be larger when near the horizon as opposed to when it's
overhead? With Venus, you normally don't see it close to much of anything
else other than other stars. During a transit you have the Sun right there
for a relative comparison.

Also being silhouetted by the Sun and being a dark object seeing against a
brighter object is there some illusion that adds to the fact that Venus is
as close as it get to the Earth, and together this makes Venus appear larger
than expected?

I know in the world of art, that placing a light object against a dark
object or visa versa can make a big difference in the way an observer
perceives a shape.

Just some thoughts,
Francis Marion