Not only does VLBI demonstrate that Sagittarius A* is very heavy, it
also indicates that Sagittarius A* is extremely compact. Usually VLBI
has a very high resolution, alas, the radio light coming from
Sagittarius A* passes through some nasty ionized medium in the Galaxy
which blurs the image. If one goes to higher frequencies the blurring
becomes less strong and the resolution of the telescope increases.
You can see how dramatic this effect is if you go from 5 GHz to 43 GHz
(Figure: The size of Sagittarius A* as it would be seen with VLBI when increasing the frequency, here the sequence is 5, 8, 15, 32, and 43 GHz. We show Gaussian modelfits to the data.)