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 .