This all seems completely reasonable to me.Exaggerated physical gestures in acting are often rooted in a theatrical rather than cinematic background (effectiveness and quality of the exaggeration varies of course). Less the case today (what with mics and projection screens a frequent presence in larger venues), actors of Shatner's generation were trained to project to the "cheap seats" (while those down front knew that such projection was to be expected). When working in film and television, such actors had to tone things down considerably--some were better than others. Even the good ones (and Shatner could be quite good) usually did their best work in the hands of a skilled director (or at least one who could override the actor's propensity for hamming it up--though even Kubrick could not entirely reign two giants (Olivier and Laughton) in Spartacus.) I have often suspected that Shatner's lesser performances, in Trek or elsewhere, owed as much to a lack of strong direction to his opinion of the script. This is true of MANY actors, but particularly of classically trained stage actors.
It occurs to me, thinking about this, that the black-and-white TV with dodgy tuning that I sometimes had to watch TOS on back in the day, on UHF during 1970s syndication, certainly constituted the "cheap seats" of television viewing in that era; Shatner reached me through that.