I think there's a misunderstanding here about Pluto's history. They didn't "downgrade" it to dwarf planet because they found it was smaller than they thought -- at least, not directly. There was a whole generation between those events. After Pluto was first discovered in 1930, estimates were taken of its mass and it was deemed large enough to be called a planet, though still quite small. In 1978, its moon Charon was discovered and it was realized that Pluto was even smaller than we thought. After that, a number of scientists questioned whether something so small really should be counted as a planet, but the question wasn't really dealt with in earnest until 2005, when we discovered the body now called Eris (then nicknamed "Xena") and determined that it was more massive than Pluto. That forced the issue: if Pluto was a planet, then so was "Xena," and if "Xena" wasn't a planet, then neither was Pluto. This led to the infamous 2006 International Astronomical Union conference that led to the formal definition of the dwarf planet category which includes Pluto, Eris, Ceres, and the trans-Neptunian objects Haumea and Makemake, and which is expected to expand to include dozens or hundreds of other objects whose mass and shape haven't yet been measured precisely enough to define their status.
So contrary to popular belief, the debate was never really about what to call Pluto; it was about what to call Eris and other possible future objects that we might find to be larger than Pluto. Finding a systematic definition that could tell us what label to put on those objects necessarily affected Pluto's status as well, but the general public's fixation on Pluto as the one and only subject of the controversy is completely missing the point.
