To effectively answer this, you gotta dig down to laser basics - if you limit the aperature size, all it would do is limit the number of excited photons exiting the laser. Lasers work (very basically) by a photon bouncing off of excited electrons - which charge that photon, and emit a clone of the photon as well. the photons can be emitted in almost any direction, but if a photon hits the electron at a specific angle, it also exits (along with its' clone) at the same angle. all the now highly charged particles of light are going in one direction, maybe even bouncing off of 2 mirrors - one with 100% reflectivity, and another that reflects some, but allows others to pass - thus continuing the chain reaction inside the "lasing" chamber. The photons that come out, can then pass through colliminating optics, filters, etc. BUT there is no way to "squeeze" those photons in order to speed them up.
Long winded, eh ? Anyways, I left out some stuff, and put other stuff in my own order, but that is most likely as simply explained as is possible. I read this on some site that actually went into great detail (on a low-level) to explain it to 8th graders, I think ! Pretty good read - if I can find the link, I'll post it.