Note 70 — Chapter XXIX · SOPHISTES — POLITIKUS.
Reading edition
Plato, Sophist. p. 257 B. Ὁπόταν τὸ μὴ ὂν λέγωμεν, ὡς ἔοικεν, οὐκ ἐναντίον τι λέγομεν τοῦ ὄντος, ἀλλ’ ἕτερον μόνον . . . Οἷον ὅταν εἰπωμέν τι μὴ μέγα, τότε μᾶλλόν τί σοι φαινόμεθα τὸ σμικρὸν ἢ τὸ ἴσον δηλοῦν τῷ ῥήματι.
Plato here means to imply that τὸ σμικρὸν is the real contrary of τὸ μέγα. When we say μὴ μέγα, we do not necessarily mean σμικρόν — we may mean ἴσον. Therefore τὸ μὴ μέγα does not (in his view) imply the contrary of μέγα.