Outline doesn't matter much: these are flextural mode crystals, with the waves confined between electrodes, making some kind of... bulk shear mode wave, I think it is? And then obviously that wave is confined between boundaries so 1/2 wave is the lowest resonance, and n+(1/2) wave overtones beyond that.
The outline (of electrode and wafer) control how waves from the local area spread out and reflect around; reflections will give spurious tones, probably of poor coupling for the most part (narrow spectral lines, low amplitude), and mode conversion to longitudinal, surface-acoustic and beam modes will give all other manner of tones, mostly at frequencies well outside of the oscillator bandwidth, but occasionally splitting poles, i.e. making resonances near the main mode, or when nonlinearity is included so that once the main oscillation starts up, other modes can potentially mix in (good luck probing those, lol, and the oscillator doesn't care once started up on the main peak, but to say in principle at least?).
You can imagine something like... if you find some spur modes that couple into surface waves, and then you start the cantilever oscillating (1/4, 3/4, etc. wave as the case may be), you've got modes on top of modes and could get some FM (manifests as spurs splitting into peaks), analogous to the warpy-woobly sound of a sheet of metal being flexed around, but in specific controlled modes and rates.
The bulk shear mode of course is largely immune to such effects, so is a good choice as dominant oscillation mode.
In any case, all those spurious modes will vary with how much space they have to fill, and the ratio between OD (outer diameter, or outer dimensions) and overlapping electrodes (dia/dimensions) will have something to do with the relative amplitude, spacing and abundance of those modes.
Tim