Vlastos on Reasons and Causes

Why is x F?
εἶναί τι ἕκαστον τῶν εἰδῶν καὶ τούτων τἆλλα μεταλαμβάνοντα αὐτῶν τούτων τὴν ἐπωνυμίαν ἴσχειν (Phaedo, 102a10-b2)
Each of the Forms exists and other things acquired their name by having a share in them.

The 1st part of this statement (claiming the safe but uninformative αἰτία):
∀F∀x∃ϕ (Fx ↔ x participates ϕ)
ϕ: Form, x: individual, F: immanent character designated by adjective or abstract noun.

The 2nd part of this statement (claiming the clever αἰτία):
Instead of mentioning just one Form, ϕ, Socrates now refers us also to another, Γ, so related to ϕ, that whatever is “named after” Γ, will also be name after ϕ (103e-104b). X is F because it participates in Γ and Γ entails ϕ.

Phaedo 100b-102e

Isn't Simmias shorter than Phaedo and taller than Socrates "by a head"? Why does Plato insist that "a head" cannot serve as a proper explanation of the above phenomena?