  Massive Galactic clusters (> 1000 Msun) exhibit a clear correlation between
cluster density, size and age and can be sorted in two categories, i.e.
starburst and leaky clusters. The reason for the existance of two types of
massive clusters is an open question. However, the answer is probably connected
to a different formation histories of the two types. In this study we
concentrate onleaky clusters only and investigate possible formation scenarios
and gas expulsion phase. This is done by using existing observational data and
numerical results of embedded cluster properties. Assuming that a clear
correlation between cluster density, size and age exists, it is shown that the
density-radius development over time forembedded clusters can be approximated
by rho \approx 100*r ^{-1.3} Msun pc^{-3}. The consequences for the star
formation process in leaky clusters are discussed and found to favour an
inside-out star formation scenario with an initially low but later accelerated
star formation rate. It is shown how the leaky clusters form in a unique
sequential manner and that rapid gas expulsion is responsible for the 80-90\%
mass loss over the next 20 Myr.
