1. The face is done well, with the classic Zeugma style with a lot of triangles being used.
2. Now look at the position of the head on the shoulders, not quite central is it?
3. His left arm (A) is out of proportion, shoulder to elbow is much shorter than elbow to wrist. Look also at the shape of elbow to wrist, also there doesn't seem to be a wrist.
4. Top of thigh to knee (B) should equal knee to base of foot yet is longer, not by much but enough to be noticeable.
5. His right leg, (C), the angle of his foot doesn't match in with how his thigh would lie. Also using the lighter tesserae on the folds draws the eye to see the right leg going up to his left thigh.
6. His right hand(D) isn't done to the same level of skill as his face.
This is not to deride the mosaic itself but more to show how this work could have been like a production line with different workers moving it to do different parts. Notice the difference in tesserae size between his mantle cloth and his body. You have to wonder if the head was done, then the body was done by another mosaicist then the mantle by others who weren't so accomplished in figure work.
Lastly we have to remember that these mosaics are done with the mosaicist working very close to something that is designed to be seen from standing position. ideally you need to keep stepping back from your work to check how it appears. That's fine if you're working at a table but not so easy if you're on your knees or sitting on the floor!
So when you look at figure mosaics look at them from an anatomical perspective as well as stylistic impression.