This isn't the first time I've heard this question. I think the crowns on the castle are stopping people's eyes from going all the way up. It's still the same 77 feet from moat to the top of the tallest spire.
When they decorated Cinderella Castle for Walt Disney World's 25th Anniversary celebration, this same question came up. The castle looked smaller. I guess it was the loss of the forced perspective. Although Sleeping Beauty Castle isn't decorated up like a birthday cake, I guess the crowns/tiaras might make it look smaller.
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