קטן qatan "make smaller, diminish". The opposite of גדל cf. Am 8:5.
The adjective קָטֹן qaton "small" is a very common word, in Amos 6:11 it speaks of the "little" houses of ordinary folk (by contrast with the "great houses" of the elite) . While in 7:2 and 5 he speaks of Israel (Jacob) as so "small" that he will not survive the coming punishments.