When and why did astronauts say Pluto was too small?

Answer 1

The International Astronomical Union (IAU) formally decided that Pluto was no longer a major planet at its annual meeting, on August 25, 2006.

One of the criteria given was that a major planet has 'cleared the neighborhood of its orbit', meaning that it is gravitationally dominant enough to have removed or controlled any objects that share its orbit. For example, Earth controls the orbit of the Moon, and Jupiter controls the orbit of its thousands of Trojan asteroids.

Pluto, on the other hand, not only hasn't cleared its neighborhood, but it is in fact gravitationally bound to Neptune, sharing what's known as a resonance orbit. This is what ultimately led to its demotion (and at the same time, Ceres' promotion to minor planet status, as the largest of the asteroids).