Qualitative arguments and examination of numerical data for 1641 organic molecules are used to establish semiquantitative relationships between the size of a molecule and some of the moments of its electron momentum density. The peak height of the Compton profile correlates well with molecular size and volume. Surprisingly, the molecular volume enclosed by the 0.001 a(0)(-3) isodensity contour of the electron number density can be predicted, with a mean absolute error of 2.1%, from (1/p(2)). (C) 2014 Elsevier B.V. All rights reserved.