Their smaller size may be attributed to their home world and radioactivity levels. Which could over generations cause stunted growth. Also black skin can be caused by generations of a people exposed to sunlight. So I feel the skin colour isn't a huge trait on a successor chapter. ... The red eyes though. Definitely more of a hint at their gene seed origins!
I also think of all the chapters, the Salamanders are on of the most worthy to be legion sized due to their kindness to the rest of humanity.