Al-mathānī here identifies a traditional group described as generally having fewer than one hundred verses. The technical context matters; every other use of the Arabic word should not automatically be assigned the same meaning.
Al-mufaṣṣal is the familiar sequence of shorter surahs with frequent boundaries. Such surahs may still contain elaborate images of the Last Day, compact affirmations of monotheism, striking oaths, or demanding moral instruction.
Size versus purpose: fewer verses do not indicate that a surah is conceptually slight. The groupings describe relative length and placement, not the degree of significance or complexity of the guidance expressed.