Part of this is due to the small surface area of massive type corals. They grow from within. Certainly, though, the tissue expansion during the day can be surprising and alludes to a much larger coral than actually exists by measure of the skeleton. Favia are one of several massive type corals who seasonally add strontium to their skeletons in ratios that approach or exceed natural sea water levels. The substitution of strontium for calcium in their skeleton at such yearly times has allowed for fairly accurate ages to be determined for colonies in the wild.
Strontium also results in a denser skeleton.
I am unsure of the method, but in situ observations of my own colonies shows that a translucent thick gelatinous tissue or secretion seems to exude from the porous skeleton underneath. This seems to act as a cement that eventually leaves the colony permanently attached to the substrate. I would be interested in hearing if anyone has more information on this phenomena. In terms of disease, Favia is quite resistant. Perhaps this is due to the fact that the tissue lies deep within the corallites and a large portion of living tissue is relatively unexposed to irritants and external noxious stimuli and pathogens.