Context : The classic question that how young massive star clusters attain their shapes and sizes , as we find them today , remains to be a challenge .
Both observational and computational studies of star-forming massive molecular gas clouds infer that massive cluster formation is primarily triggered along the small-scale ( \lesssim 0.3 pc ) filamentary substructures within the clouds .

Aims : The present study is intended to investigate the possible ways in which a filament-like-compact , massive star cluster ( effective radius 0.1-0.3 pc ) can expand \gtrsim 10 times , still remaining massive enough ( \gtrsim 10 ^ { 4 } M _ { \odot } ) , to become a young massive star cluster , as we observe today .

Methods : To that end , model massive clusters ( of initially 10 ^ { 4 } M _ { \odot } -10 ^ { 5 } M _ { \odot } ) are evolved using Sverre Aarseth ’ s state-of-the-art N-body code NBODY7 .
Apart from the accurate calculation of two-body relaxation of the constituent stars , these evolutionary models take into account stellar-evolutionary mass loss and dynamical energy injection , due to massive , tight primordial binaries and stellar-remnant black holes and neutron stars .
These calculations also include a solar-neighbourhood-like external tidal field .
All the computed clusters expand with time , whose sizes ( effective radii ) are compared with those observed for young massive clusters , of age \lesssim 100 Myr , in the Milky Way and other nearby galaxies .

Results : It is found that beginning from the above compact sizes , a star cluster can not expand by its own , i.e . , due to two-body relaxation , stellar mass loss , dynamical heating by primordial binaries and compact stars , up to the observed sizes of young massive clusters ; they always remain much more compact compared to the observed ones .

Conclusions : This calls for additional mechanisms that boost the expansion of a massive cluster after its assembly .
Using further N-body calculations , it is shown that a substantial residual gas expulsion , with \approx 30 % star formation efficiency , can indeed swell the newborn embedded cluster adequately .
The limitations of the present calculations and their consequences are discussed .