Using perturbation theory, expressions are derived for the frequency shift of a circularly polarized resonant microwave cavity upon insertion of a ferrite sample small compared with the wavelength. Retardation effects are considered in deriving the rf field h2 internal to the sample. By integrating h1·h2dv (where h1 is the applied rf field) over the sample volume, a closed‐form expression is obtained for the cavity frequency shift. The theory is applied to the magnetic resonance in ferrites. The calculated magnetic resonance line shape is found to remain symmetrical as the sample size is increased. Retardation effect, thus, does not explain the asymmetry of the resonance lines which have been observed for polycrystalline ferrite. Results of the theory are also compared with measurements on a single crystal of ferrite having the rather large maximum permeability loss term of 100. Theoretical and measured values are in close agreement. The theory is also compared with the somewhat different results of other recent theoretical treatments, in which an approximate expression for an effective magnetic dipole moment for the sample was computed, rather than integrating the internal rf fields over the volume of the sample.