I think it's some of the following
-Perspective, his perspective is unique to most other portrait artists, that it felt more like it's flowing in the air, rather than the eye of the artist sitting on a stool.
-Dimensions. They're always perfect in his portraits, the ratio of the subject to the background, it's always perfect. His full body portraits especially showcase this.
-Contrast and palette. He knew how to make the subject pop, and his portraits are slightly more vibrant than most other aritsts. Not sure if it's because they're better preserved and/or better scanned or by design.