This is an excellent question for the first post--welcome.
Usually, the SD mics are of a single diaphragm and LD are of dual one (so called Braunmuhl-Weber design). Besides the fact the LD has much more resonant modes (esp. edge terminated) and as a consequence more colored sound, both have different construction. I'd suggest to read this excellent paper from folks from Shure:
You might need to read it a few times (just skip all the math if you are not into that stuff), but if you really want to understand the processes in depth, I know no better source.
wow - nice paper from Shure - hadn't seen that before -- on it
Dewi - the "How does diaphragm size/polar pattern relate to mic applications?" sticky thread at the top of the mic forum is a great place to learn more than you thought there was to know on this subject, also. It's long (and you can skip all the posts by me, if there are any but definitely read posts by Harvey Gerst, and by Marik, if there are any), but the concepts are expressed in ways that I found easy to understand.