The two "biggest" no pun intended reasons are:-
The are more "bigger guys" than there were 20 years ago and thus we've become more used to them. When Hogan debuted there was him, Studd, Andre at those big heights with a few around 6ft 5" like Barry Windham... the giants would be replaced from time to time with guys like One Man Gang or the like but you never had too many "big men" of that type on the roster... they were still relatively rare in the business. Around 1990 it began to explode, more taller guys began to choose wrestling. By 1996 the WWF had Kevin Nash, The Undertaker, Glen Jacobs, Kama/The Godfather, Sid, Goldust, Scott Hall, Erik Watts, Dennis Knight, Duke Drose AND Mabel and the Harris twns all on the roster over 6ft 5" with a few like Mark Canterbury, JBL and Bart Gunn around the 6ft 4 mark... basically EVERYONE was huge and within a year they were adding Kurgann, Giant Silva to that as well...
Bigger guys are not an unusual thing any more so it means when we see them it's not special...also the Rey thing, where a tiny guy has been pushed means it's almost immaterial now what size a wrestler is in terms of height, it's almost desensitization.
The other thing is of course HD. A lot of the tricks they used to use to make guys "seem taller" went out the window... Kane doesn't wear lifts cos it'll show on HD that's what he's doing which then equalises out his height to others on the roster.
They basically had to change how they present the entire programme when HD came along, the "bigger guys" outside of those we're used to seeing like Barrett for example seem bigger than they did in the previous format... while smaller guys don't seem as small. It's a particular quirk of HD WWE can use that wasn't there before.. but it means even the really big guys don't seem as big anymore.
Is it a bad thing? No... it's not always as realistic but the old Bret Hart adage is true "once you're on the mat you're the same size..." and that worked for Jericho to his advantage when he fought Goldberg for real.
