Scaling Brain Size, Keeping Timing: Evolutionary Preservation of Brain Rhythms by György Buzsáki, Nikos Logothetis and Wolf Singer
In this paper, they report changes of frequency of several rhythms as a function of brain weight. What I found interesting was Fig. 2 showing that most rhythms are invariant to brain weight (or equally size) except theta, suggesting that spatial properties of the hippocampus (e.g. its overall size) indeed constrain the range in which theta rhythm emerges.