Poirier. Same size or bigger.
Holloway. Conor was probably a little bigger, but again, nothing too crazy. Holloway also said that he will go to LW.
And it's not like he has a crazy advantage against the rest. Jones has a crazy advantage against Cormier.
Conor - 5'9" with 74" reach
Brimage - 5'4 with 71" reach
Holloway - 5'11" with 69.5" reach - Max is a twig next to Conor, at least 5lbs of muscle difference
Brando - 5'7" with 69" reach
Dustin - 5'9" with 72" reach - Conor has a reach advantage, and was probably at least 2-3lbs heavier when they fought
Siver - 5'7" with 70" reach
Mendes - 5'6" with 66" reach
Aldo - 5'7" with 70" reach - Worth noting that Aldo was commonly thought to be the heaviest FW, but was at least 2-3lbs smaller than Conor, as well as shorter and has less reach. He's smaller in every metric.
So yes, Conor is bigger than everyone else at 145. It's a fact. The only person taller than him was Max, and he's not as heavy, or rangey. The only other person close is Dustin and he has a shorter reach, and was a tad lighter.
Point stands - when outside of 145, his size advantage disappears completely, and with Nate in particular, he's no longer the Southpaw.