that happened in that chamber happened twice, echoing. However the beam may have been rendered, there's no reason to doubt that it was echoing in time the same way as every other motion and line of dialogue. The apparent thinness of the original beam relative to the second beam was most likely an illusion. The beam grew dimmer toward the edges; the first iteration of the beam was dimmer overall, half-faded out, and thus its edges were dim enough not to stand out from the background illumination (though, checking a screencap of the shot at TrekCore
, I can see them faintly). But when the whole beam was seen more brightly and clearly, then the edges were bright enough to stand out against the background, thus creating the illusion that the beam was wider. It's pretty common for the human eye to perceive brighter lights as "spread out" more than dimmer lights because of this effect.
Just to verify it, here's the original episode script
Archer hears the footsteps again, but this time he sees
something --a slight DISTORTION against the far wall. He
quickly FIRES! A blurred PRE-ECHO of the shot precedes the
Silik FIRES the pistol! The PRE-ECHO HITS Archer square in
the chest, but before the real beam can reach him, Archer
DARTS to his left and the blast MISSES him by inches!