1. Most sports don't have weight classes or size divisions. You dance with the body that brung ya. Think it's no fair that you can't be an offensive line man for the Broncos because you're only 165 lbs? Tough cookies. That linebacker is coming. If you can't stand in the way and stop him, you aren't going to make the field. Only fight fans think it's 'unfair' to be bigger than the athletes you compete against.
2. It's around there, though. That's pretty clear. What's not clear is why some fight fans think it's entirely fair that a guy who weighs 157 lbs should be matched up against a guy who weighs 170 lbs, but entirely unfair that a guy who weighs 172 lbs should be matched up against a guy who weighs 170 lbs. The weight cutoffs are arbitrary, but the classes have a range. Fighters aren't meant to be the exact same size. They're meant to be within 15 or 20 lbs of one another (10 lbs at the lighter classes). And that's exactly what happens at least 95% of the time.
3. Not looking for yellow cards, so I won't spell it out to blatantly, but if you think these guys are smaller on fight night because they're fighting 10-15 lbs dehydrated, you may be a little drunk.