Why is musāwāt called “the baseline of comparison”?

Musāwāt and Ījāz al-Qaṣr: Proportionate Wording and the Rhetoric of Qiṣāṣ

• Concision and expansion can be compared with proportionate expression
• Because eloquence must always use musāwāt
• Because musāwāt always fixes a numeric word count
• Because it is confined to declarative statements