  Using hydrodynamical simulations of entire galactic discs similar to the
Milky Way, reaching 4.6pc resolution, we study the origins of observed physical
properties of giant molecular clouds (GMCs). We find that efficient stellar
feedback is a necessary ingredient in order to develop a realistic interstellar
medium (ISM), leading to molecular cloud masses, sizes, velocity dispersions
and virial parameters in excellent agreement with Milky Way observations. GMC
scaling relations observed in the Milky Way, such as the mass-size ($M$--$R$),
velocity dispersion-size ($\sigma$--$R$), and the $\sigma$--$R\Sigma$
relations, are reproduced in a feedback driven ISM when observed in projection,
with $M\propto R^{2.3}$ and $\sigma\propto R^{0.56}$. When analysed in 3D, GMC
scaling relations steepen significantly, indicating potential limitations of
our understanding of molecular cloud 3D structure from observations.
Furthermore, we demonstrate how a GMC population's underlying distribution of
virial parameters can strongly influence the scatter in derived scaling
relations. Finally, we show that GMCs with nearly identical global properties
exist in different evolutionary stages, where a majority of clouds being either
gravitationally bound or expanding, but with a significant fraction being
compressed by external ISM pressure, at all times.
