  The magnification induced by gravitational microlensing is sensitive to the
size of a source relative to the Einstein radius, the natural microlensing
scale length. This paper investigates the effect of source size in the case
where the microlensing masses are distributed with a bimodal mass function,
with solar mass stars representing the normal stellar masses, and smaller
masses (down to $8.5\times 10^{-5}$M$_\odot$) representing a dark matter
component. It is found that there exists a critical regime where the dark
matter is initially seen as individual compact masses, but with an increasing
source size the compact dark matter acts as a smooth mass component. This study
reveals that interpretation of microlensing light curves, especially claims of
small mass dark matter lenses embedded in an overall stellar population, must
consider the important influence of the size of the source.
