One of the bulkiest WW's around? Not even close..
Diaz was definitely bigger
Can't see the picture, but if it was a weigh in pic it doesn't really mean much since they both weighed the same at that moment so Diaz would obviously look "bigger" since he is taller and has longer arms.
When they actually fought Cyborg was more muscular, he had a bigger chest, bigger back, bigger traps, bigger arms etc. and I bet he was the heavier of the two. The guy started his career at LHW and spent much of it at MW, look at how cut Cyborg is at MW compared to what Diaz looked like when he fought Shamrock at 180. Plus Diaz has made 160, I don't think Cyborg could do that (and if he could it would be very, very difficult).
Diaz is taller and has longer arms, that's it.