I do agree with bigger = better, but I am not sure I agree with the relationship between "Bigger reflector = more light on the edges thrown forward = beam goes farther, a thrower Smaller reflector = more light on the edges spills out = beam goes wider, a flooder"
Not having done the math but my Physics sense tells me:
It is a choice between the efficiency (light transmission efficiency vs light reflection efficiency) and practicality (cooling/assembly) of the reflector vs the refractor vs the dollar.
In a nut shell, larger head is more efficient but more expensive. Here is why:
To project a perfectly parallel beam (a constant size circle regardless of distance), the lens needs to be exactly one focal length from the "point source" emitter. But this math is complicated by the lens on the emitter itself (and it isn't a point but likely a square). So I would go with the mere Physics "sense" for now.
For an evenly distributed circle of light, a large reflector creates a larger "source" of light (make the emitter seem farther) allowing for longer focal length refractor (thinner lens). A smaller reflector will require a shorter focal length refractor (thicker lens). So, thinner "lens" with bigger reflector, vs thicker lens with small reflector. (Actually, it is not the thickness everywhere, but how convex it must be. So it is how much MORE thick the center must be vs edge thickness.)
Reflective is "free" (from efficiency standpoint). If your reflective material reflects 80% of the light, however big a reflector, each and every spot on the mirror you get 80% efficiency. The size does not affect efficiency.
Refractivity is expensive. The thicker your lens, the more light is lost as it passes the refractive (lens) material.
Common sense that price increase with size of mirror. Also, price of the lens increases more by diameter than it's increase due to thickness. In other words, a thick small lens is a lot cheaper than a thin big lens.
So, big=more expensive to make (beyond the cost of material) but should be more efficient.
Rick