Did you just assert that the size difference between Jon Fitch and B.J. Penn is the same as the size difference between Brock Lesnar and Randy Couture?
Penn, at lightest, will step in at 165 pounds (a hugely low number) and Fitch will step in at (as we've established as a high estimate for the largest welterweights) 190 pounds. That's 25 pounds I'm spotting you, generously.
Compare that to the weigh in results from Couture vs. Lesnar, not even considering the weight that Lesnar gained after rehydrating, and you have a 45 pound weight difference. Couture weighed in at 220 and Lesnar at 265.
So, there's a 20 pound difference between those two fights. So, no, they're not the same.
I know I am late to the game but I still think Fitch is bigger than 190 in the cage. BJ being 165 gives IMO at least a 30lb weight difference. When you consider that its 165 pound guy giving up 30lbs or 18% of his body mass and Couture gave up like 20% it is in the realm of comparison at the least.
Your opinion that fighters don't cut that much baffles me I was at even a local fight last week and sitting right were the fighters get checked before entering the cage and there was a WW from Kelowna BC who was bigger than me I am am 6'1 192lbs. He had to be at least 200lbs if not more to be honest I was mind boggled because it was such a small card I couldn't figure out why somebody would do that to themselves. (most guys on the card looked like they cut maybe 10lbs.