First of all, the statement that "pseudo coelomates have very minute width of their body cavity and rest of the body cavity is filled with mesoglea" is not correct at least for (large) nematodes and priapulids: they have quiet voluminous body cavity and no mesoglea. In small nematodes (to which Ascaris doesn't belong) the body cavity is indeed vestigial, but the "space" between the organs is filled with extracellular matrix.
Another point is that the eggs in nematodes develop not in the body cavity directly, but in the ovaries and uteri (as correctly shown in the picture you provided), which indeed occupy very large part of the body.
Now to the question about the counts: individual capacity of a single female 29 cm long was once (see also) estimated to be ca. 27 mln eggs contained in its ovaries and uteri, which is impressive, but reasonable given the extremely small size of the eggs and extremely large size of the worm.
If we very coarsely approximate the volume occupied by a single egg with a cube 70x70x70 µm3, we get ca. 9 ml for 27 mln of them, and not much imagination is required to see, how a female ascarid can find room for this volume of relatively loosely packed eggs in its genital system.
To check my statement about "quiet voluminous body cavity and no mesoglea" one can open any modern textbook on invertebrates (or attend a practical course of invertebrate zoology). This is how the nemotode body cavity is described in the monograph "Nematoda" by Vladimir Malakhov (the original I cite is in Russian, but there is also a translation of the book):
- "The presence of a spacious body cavity is considered to be the most characteristic property of the whole class of nematodes".
- "Schizocoel [= primary body cavity] doesn't have its own walls made of cellular material and is represented by spaces between organs filled with liquid".
- "Nevertheless, it should be stressed, that the opinion of the schizocoel being a voluminous body cavity, appeared as a result of investigations on large parasitic nematodes."