In the early days of the sport fighters like Royce Gracie proved that technique can over come sheer size and strength. but in modern mma almost every practitioner is skilled, especially at the top level, so this is when size does become important.
cutting weight has become a necessity because otherwise your opponent could have a huge advantage. where most guys cut between 10-15lbs there are a few who cut monstrous amounts of weight. its treated as a science to some, sweat off 20lbs+ and rehydrate correctly to ensure your body gets back all its electrolytes and you can fight at 100%.
below is a list of fighters who i think are the best weight cutters in MMA (or at least ufc)
5. Nate Marquardt
many people claim that Anderson Silva cuts an unfair amount of weight to get to mw, but in my opinion Nate Marquardt is even bigger. he's 6 ft, and packs on more muscle than anyone else in the weight class. maybe Palhares has more muscle but he's alot shorter.
4. Thiago Alves
This man was getting alot of bad press for the amount of weight he cuts a couple years back. he was 3lbs overweight against hughes and many thought he was too big to make the weight anymore. he was injured leading up to the hughes fight which affected the amount of weight he could cut, but he's made weight in every fight since. he reportedly steps into the cage at 200lbs which is monstrous for a WW.
3. Gleison Tibau
At first glance you'd think this man was a fairly large WW, well actually he cuts all the way down to 155lbs. i couldn't find any hard evidence but his wikipedia profile suggests he steps into the cage at 183lbs, thats not far off GSP's fighting weight!
2. Anthony Johnson
Anthony Johnson cuts the most weight by far, i'd have him as NO1 going from stats alone but i have my reasons for keeping him at 2nd place. A couple months before the koschek fight he was walking around at 220lbs, that makes him a natural lhw! he obviously wasn't cutting 50lbs of water weight before the fight but you can be damn sure he steps in the cage over 200lbs.
1. Brock Lesnar
Brock is what defines weight cutting. he's a mammoth of a man and has to cut quite a bit to make the huge 265lbs limit of the hw division. it's because of him that frank Mir and over hw's are becoming obsessed with gaining size. pretty soon they'll have to make a 220lb division, all because Brock came in and dominated with his brute size.