Point lamps tend to be less noisy because they're discrete. I.e. the ray either found the lamp, or it didn't, whereas with a mesh light, a ray can find one of any number of different parts of the mesh light, each of which will produce a slightly different result.
If you're familiar with the single-double-slit experiment in physics, it might help make more sense. If you're not, I can explain very briefly: think of a machine gun randomly spraying bullets at a bullet-proof wall that has a single vertical slit in it. Then imagine the pattern that the bullets will make if they are able to go through the vertical slit and hit the back wall.
If you have a very large slit, then the pattern on the back wall will be large and indistinct. There will be generally more bullets in the middle and it will fade off toward the edges (where bullets are less likely to hit). It will also take a LOT of bullets to fill in that area behind the open slit. If you have a very narrow slit, then the few bullets that are able to make it through will make a much more distinct pattern/line on the back wall.
(Warning: very simple explanation following -->) When Cycles is trying to determine the color of a point on a surface, it sprays rays (like bullets) to see what they hit. If they hit a light source, then the surface is illuminated. The light source is a little bit like the slit. If it's very large and easy to find, then it will take a lot of rays to resolve the pattern. If the light source is small, then the few rays that hit it will be exactly right, and the pattern will converge more quickly (because rays that don't hit the light are relatively easy to discard).
SO, long story short, if you have point light source, then it's like having a slit that is exactly as wide as the bullet going through it. This means that you only need a few bullets to make it through the slit before you get a nice clear line on the other side. With the point light source, you need fewer rays to hit it, so it isn't as noisy.
That being said, the discrete nature of point lights makes it really hard for them to give nice soft shadows. The "size" of the point light is basically a way for Cycles to make a point light give soft shadows by making it bigger...like a mesh light.
Computationally, there might be something about point lights that makes them easier to calculate, even though they have "size," but I don't know anything about that side of things.
Hope that helps!