They have changed the weight requirements at least twice in the WWE.
Cruiserweight and Light Heavyweight (What the WWF championship was called before they bought WCW) are both weight designations in the style of real wrestling and boxing (Which is usually faker than a Hulk Hogan match). Weight classes have different championships/titles/divisions so that nobody has a real huge advatantage simply because of weight.
In wrestling, it doesn't matter, because you have cruisers with heavyweight title shots and a floating weight limit for whatever's convenient. It's optional in pro wrestling.
Not everyone who IS a cruiserweight is required to compete as one. For example, it's been reported for a while that Rey thinks he's bigger than the CW division.
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