He's a lot shorter than Lan Sizhui, and softer around the edges. Where Lan Sizhui is all muscle, Ouyang Zizhen has been filling out slightly, making his face look rounder and giving him a gentler, cuter look.
Unable to help himself, Wei Wuxian pinches his reddened cheek. This might be a little irrelevant right now, but don't you think it was a sweet part?