What is the physics behind the Larson mass-size relation?
  Different studies have reported a power-law mass-size relation $M \propto
R^q$ for ensembles of molecular clouds. In the case of nearby clouds, the index
of the power-law $q$ is close to 2. However, for clouds spread all over the
Galaxy, indexes larger than 2 are reported. We show that indexes larger than 2
could be the result of line-of-sight superposition of emission that does not
belong to the cloud itself. We found that a random factor of gas contamination,
between 0.001\%\ and 10\%\ of the line-of-sight, allows to reproduce the
mass-size relation with $q \sim 2.2-2.3$ observed in Galactic CO surveys.
Furthermore, for dense cores within a single cloud, or molecular clouds within
a single galaxy, we argue that, even in these cases, there is observational and
theoretical evidence that some degree of superposition may be occurring.
However, additional effects may be present in each case, and are briefly
discussed. We also argue that defining the fractal dimension of clouds via the
mass-size relation is not adequate, since the mass is not {necessarily} a proxy
to the area, and the size reported in $M-R$ relations is typically obtained
from the square root of the area, rather than from an estimation of the size
independent from the area. Finally, we argue that the statistical analysis of
finding clouds satisfying the Larson's relations does not mean that each
individual cloud is in virial equilibrium.
