  Most stars form as part of a star cluster. The most massive clusters in the
Milky Way exist in two groups - loose and compact clusters - with significantly
different sizes at the end of the star formation process. After their formation
both types of clusters expand up to a factor 10-20 within the first 20 Myr. Gas
expulsion at the end of the star formation process is usually regarded as only
possible process that can lead to such an expansion.We investigate the effect
of gas expulsion by a direct comparison between numerical models and observed
clusters concentrating on clusters with masses >10^3 M(sun). For these clusters
the initial conditions before gas expulsion, the characteristic cluster
development, its dependence on cluster mass, and the star formation efficiency
(SFE) are investigated. We perform N-body simulations of the cluster expansion
process after gas expulsion and compare the results with observations. We find
that the expansion processes of the observed loose and compact massive clusters
are driven by completely different physical processes. As expected the
expansion of loose massive clusters is largely driven by the gas loss due to
the low SFE of ~30%. One new revelation is that all the observed massive
clusters of this group seem to have a very similar size of 1-3 pc at the onset
of expansion. It is demonstrated that compact clusters have a much higher
effective SFE of 60-70% and are as a result much less affected by gas
expulsion. Their expansion is mainly driven by stellar ejections caused by
interactions between the cluster members. The reason why ejections are so
efficient in driving cluster expansion is that they occur dominantly from the
cluster centre and over an extended period of time. Thus during the first 10
Myr the internal dynamics of loose and compact clusters differ fundamentally.
