Top 大 is the entry's indexing radical and pictures a person with arms spread wide. Sitting above 镸 (long) it hints at the wide-and-long covering being pulled on or laid over something, the original sense of 套 to encase, sheathe, set.
Lower 镸 is an older graphic variant of 长 long. Below the wide 大 it doubles the long-and-spread imagery, reinforcing the meaning of a covering or set that wraps fully around its object: sleeves, harnesses, suites of furniture.
