This question concerns only written sentences, and presumes that longer sentences are more difficult to understand, a presumption which consists with the formula behind the Flesch–Kincaid readability tests (which I discovered from the first paragraph hereof).
What specific capabilities, knowledge, or skills are needed to ameliorate understanding of longer sentences (which were more common in the past in English and the Romance languages)?
What empowered olden people to communicate comfortably with longer sentences?
Why do we today find them more difficult?
For example, despite not having studied linguistics separately, did learning Latin and Greek impart the olden people with some linguistics that cultivated their fluency with longer sentences?