OP: Is this a figure of speech?
No, the "graded numerical sayings" are not "figures of speech".
It is more accurate to describe this "n + 1" pattern in biblical poetry in terms of "rhetoric" (or "stylistics") rather than a "figure of speech" which normally has to do with non-literal language (e.g., metaphor, simile).
Although "graded numerical sayings", as they are sometimes called, are found not only in wisdom literature (like the examples given by OP, and Proverbs 30 has a nice collection of them), but also the prophets (especially Amos 1-2), they are also found in the literature of Ugarit and in the cuneiform texts from Mesopotamia.
As another answer helpfully points out, it is often thought that the last line (the "climax" of the number of things) is the "punch-line", although there are cases like the Amos examples where the "climactic" aspect is subdued or absent. The origins of this saying-type are not certain, but the common usage suggests a kind of ancient social science, observing and classifying nature and behaviour.
An older study of this type of saying by W.M.W. Roth, Numerical Sayings in the Old Testament: A Form-Critical Study (Brill, 1965) is available with a preview on Google Books, and most newer handbooks on biblical wisdom will offer some comment on this saying-type as well.