Proust's Additions

It has previously not been feasible to study in depth the expansion of Proust's famous novel, A la recherche du temps perdu. In 1913, the novel was to be 1500 pages; by 1922, when Proust died, it was 3000. How did it grow to such proportions? Which characters were always there - which ones sprang from their author's imagination in his very last years?