The dimness has nothing to do with software, it's actually your persistence of vision.
If you draw a line, and that line has say, 10 points, the galvo's do not need to spend nearly as much time drawing it, and as such, the laser is only on for a very short time. Because of this, your eyes do not have enough time to gather the perceived brightness of the actual line, however it is really the same brightness as always.
By pasting the line over the line many times, all you are doing is increasing the time that the laser is on (More points, so it takes longer to scan), thus giving your eyes time to pick up all the light, and making it look brighter.
If you have a microcontroller and a LED, you can test this as well. Have a microcontroller turn on an LED for say, 10 microseconds. It'll look dim to you, but it'll still be at full brightness for that 10 microseconds.
Your eyes act like the long exposure setting on a camera. Longer the light is there, the brighter it is (To a point, of course)