This is definitely untrue. The British Museum alone has around 130,000 clay tablets from ancient Mesopotamia - and that's just the records from one region that are sitting in one museum's archive.
Now, granted, most tablets from ancient Mesopotamia turn out to be basically financial spreadsheets, but still, that gives you some idea of the scale we're talking about here.
EDIT: Some people below are pointing out that the professor might've meant just the text of the primary sources, printed in a small-point typeface on modern paper. This raises the interesting question of what exactly we mean when we say "primary source" - are we talking about the physical object itself, or the information we're able to extract from it?
I believe your professor may be wrong. I have done some work with Ancient Egyptian and Roman Egyptian papyri, and it seems as if there is a general consensus that there is simply too much papyri and not enough scholars! The Middle East and North Africa have had a number of digs that have led to the discovery of thousands if not tens of thousands of papyri docs (Oxyrhynchus, Nimrud, etc.) .
I should clarify that what your professor is probably referring to are transmitted literary sources, so things like Livy's History rather than tax fragments recovered from Oxyrhyncus or Sumerian cuneiform temple records. In which case yeah, it is probably true, although I should note that I am having difficulty imagining exactly how big 10x11 is. I don't think there has ever been any study of this sort, however.
hi.. you may be interested in this post which estimatesprimary source material from Ancient Rome
Not the originals. Go to somewhere like Karnak or Delphi, and the buildings themselves are the primary sources - it seems that every surface is densely covered in writing.
Kind of on the same theme. Is there a complete list of (complete / mostly complete) literary works that have come down in Greek and Latin.