Dear Rob,
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.
Cheers,
R.
