The goal of this paper is to provide evidence in favor of a phase-based account of idiom size (Svenonius 2005). The argumentation will be centered on a dative asymmetry in Spanish idioms that, to the best of my knowledge, had remained unnoticed in the literature. It will be shown that dative DPs introduced by a High Applicative head cannot be a fixed part in an idiom, as opposed to lower datives, which can indeed be fixed as part of an idiom. Building on work by McGinnis (2001) and den Dikken (2023), it will be argued that the unavailability of fixed high applied arguments in Spanish idioms is due to the phasal status of the High Applicative head. By exploiting Wood & Marantz's (2017) minimalist theory of argument structure, a novel formal definition of VP phase will be proposed that unifies the phasal status of Voice and High Applicative. Under such a unification, the well-known 'No Agent Idioms' generalization (Marantz 1997, Harley & Stone 2013) and the present discovery about datives follow from the same overarching constraint on idiomaticity, namely that an idiom cannot contain fixed material belonging to two different phases.