I have a ton of xenia elongata, all from the same original colony that I bought, and in some places in my tank some are stumpy, and right next to them, they are long and stringy. In fact, I even see combinations periodically on the same stalk. I would assume that the longer skinnier ones are seeking out food, but that's not a scientific conclusion. They all pulse, and obviously are in the same water conditions and same lighting conditions. Who knows, maybe they just like to mix it up a bit. If I get the chance, I'll photograph a stalk that has some polyps that are well over an inch long mixed in with 1/4" polyps. It could also be a factor of age, as the younger polyps seem to be shorter on these, as they are also smaller, however, the shorter polyps are the same size as the ones in the rest of the tank that are uniform. I have noticed, however, that in general, the ones that are shaded are longer, reaching for the light source IMO. But, not all of them are in that situation.