General relativity predicts that strong gravitational lensing will cause objects near a black hole to appear larger than they are when viewed by a distant observer. In the case of a Schwarzschild black hole, the minimum apparent radius of an object centered on the black hole is about 5.2 GM/c2. After accounting for interstellar scatter broadening, the apparent size of the millimeter emission is about 30% smaller than this value.
A likely interpretation of this size is that the millimeter emission arises in an inclined accretion disk around the black hole. The approaching side of the disk appears to be brighter due to relativistic Doppler boosting, while the receding side appears fainter. Since the emission is at least partially optically thin at 1.3 mm, the resulting image is smaller than the black hole and offset from its center.Doeleman, et al; Nature, v 455 pp 78-80 (2008), "Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre"
Interpreting the size of Sgr A*: (Left) Unlike in Newtonian physics, where the apparent size of an object is its intrinsic size, relativistic lensing causes objects near the black hole to appear larger than they are. The measured size of the millimeter emission from Sgr A* is smaller than the minimum apparent size of an object near a black hole, indicating that the emission seen by the EHT is offset from the center of the black hole. (Right) One possible model of the millimeter emission is an accretion disk. The approaching side of the accretion flow, seen in the upper left, is much brighter than the receding side, in the lower right, due to relativistic Doppler boosting.