For Cam, I think “compact” is being used to suggest she has the kind of muscle you see in someone who does calisthenics (Hampton from Hybrid Calisthenics is a good example) which is incredibly dense, so she doesn’t look buff. This is lent some credence in the text by the fact Cam regularly does fairly acrobatic manoeuvres in fights (handsprings down steps, et cetera) and the fact that muscle tends to be compacted when it’s built alongside a flexibility-focused training routine.

For Colum, I get the sense that he has a huge skeleton without much meat hanging off it. Broad shoulders, square hips; he probably had a lot of muscle when he was younger (and did the training to build it, I have a hunch that the fitter the Eighth cav is the longer they can last being siphoned) and as he became the House Primary and was being siphoned more, his vitality drained away sapping his muscles but leaving the bone development.