Originally Posted by Roger Hicks
Second para: yes, you are absolutely right. Ilford's epitaxial Delta grains are a lot cleverer than plain tabular grains (T-grains), which Ilford's research department tried on the way to Delta.
First para: you're right there too. Not only is there a larger surface area: a large, flat crystal can support multiple development sites.
There is a clue to yet another factor in the word "monosize". In a conventional cubic-grain film film, the crystals are of widely varying sizes, meaning that there are a lot of small, slow crystals as well as the big, sensitive ones. This is how you can use less silver and still get greater sensitivity in T-grain and Delta: far fewer "wasted" small crystals.
In practice, there's a lot more convergence between cubic and tabular crystal films than most people realize. Controlling crystal habit is one of the underlying technologies in improving both kinds of film.
In my mind's eye, it seems that the larger frontal area of each grain would reduce resolution, like having larger, but fewer pixels in a digital sensor. But in reality, Delta film seems finer-grained and sharper. Once again, there is more to this than meets the eye! Or maybe the T- or Delta-grain is not larger than conventional, just thinner?