An important fact that should be mentioned before the question is answered is that all galaxies formed at approximately the same time i.e 13 Billion years ago. The classical model of galactic formation states that galaxies formed from the collapse of matter in the form of huge gas clouds which (before the time of collapse) were larger in size than the galaxies that formed from their collapse. If the gas cloud was in a state of slow rotation at the time of collapse then it formed a longer elliptical galaxy. However, if the gas cloud was fast-rotating then it formed a spiral galaxy. A more recent model of galactic formation disagrees with the classical model and says that the large gas clouds did exist BUT instead of collapsing as is they fragmented into smaller clumps. It was these clumps that formed the galaxies. This theory explains the existence of clusters and super clusters of galaxies which is a phenomenon not explained by the Classical model. A third model of galactic formation says that galaxies form by the merging of smaller clumps the size of a million solar masses approx. The model implies that these clumps would have been able to start collapsing fairly early on the universal time line. Once these solar masses were formed it would have been easier for these masses to attract each other and move towards each other. The model also implies that there should be smaller galaxies than larger ones (an implication supported by observable fact).
