Recent work has suggested that the amplitude of the size mass relation of massive early type galaxies evolves with redshift .
Here we use a semi-analytical galaxy formation model to study the size evolution of massive early type galaxies .
We find this model is able to reproduce the amplitude of present day amplitude and slope of the relation between size and stellar mass for these galaxies , as well as its evolution .
The amplitude of this relation reflects the typical compactness of dark halos at the time when most of the stars are formed .
This link between size and star formation epoch is propagated in galaxy mergers .
Mergers of high or moderate mass ratio ( less than 1:3 ) become increasingly important with increasing present day stellar mass for galaxies more massive than 10 ^ { 11.4 } M _ { \odot } .
At lower masses , low mass ratio mergers play a more important role .
In situ star formation contribute more to the size growth than it does to stellar mass growth .
We also find that , for ETGs identified at z = 2 , minor mergers dominate subsequent growth both for stellar mass and in size , consistent with earlier theoretical results .