  Microlensing of stars places significant constraints on sub-planetary-mass
compact objects, including primordial black holes, as dark matter candidates.
As the lens' Einstein radius in the source plane becomes comparable to the size
of the light source, however, source amplification is strongly suppressed,
making it challenging to constrain lenses with a mass at or below $10^{-10}$
solar masses, i.e. asteroid-mass objects. Current constraints, using Subaru HSC
observations of M31, assume a fixed source size of one solar radius. Here we
point out that the actual stars in M31 bright enough to be used for
microlensing are typically much larger. We correct the HSC constraints by
constructing a source size distribution based on the M31 PHAT survey and on a
synthetic stellar catalogue, and by correspondingly weighing the finite-size
source effects. We find that the actual HSC constraints are weaker by up to
almost three orders of magnitude in some cases, broadening the range of masses
for which primordial black holes can be the totality of the cosmological dark
matter by almost one order of magnitude.
