  We compare the properties of giant molecular clouds (GMCs) in M51 identified
by the Plateau de Bure Interferometer Whirlpool Arcsecond Survey (PAWS) with
GMCs identified in wide-field, high resolution surveys of CO emission in M33
and the Large Magellanic Cloud (LMC). We find that GMCs in M51 are larger,
brighter and have higher velocity dispersions relative to their size than
equivalent structures in M33 and the LMC. These differences imply that there
are genuine variations in the average mass surface density of the different GMC
populations. To explain this, we propose that the pressure in the interstellar
medium surrounding the GMCs plays a role in regulating their density and
velocity dispersion. We find no evidence for a correlation between size and
linewidth in any of M51, M33 or the LMC when the CO emission is decomposed into
GMCs, although moderately robust correlations are apparent when regions of
contiguous CO emission (with no size limitation) are used. Our work
demonstrates that observational bias remains an important obstacle to the
identification and study of extragalactic GMC populations using CO emission,
especially in molecule-rich galactic environments.
