Apparent Brightness

The human eye can see light in the visible spectrum ranging from Red to Violet, i.e. the colours of the rainbow. The eye's sensitivity is non linear, so different colours are perceived at different intensities to the eye. So for the same given power, a 100mW Red laser will appear much dimmer than a 100mw Green laser.

The colour the human eye is most sensitive to is a shade of Green which has a wavelength of 555nm.
DPSS Lasers produce a lime green which has frequency of 532nm, which is very close to the peak of the eye's sensitivity. The greens and blues produced by a typical argon are further away from the peak response of the eye, so they appear to be dimmer in comparison.

This is why DPSS laser are said to be 3 to 4 times brighter than a typical argon laser. So a 100mW DPSS laser will look the equivalent to a 300mw argon laser in terms of apparent brightness.

Taking this to an extreme, if you were to compare a 100mw 532nm Diode laser with a 2W Infrared Laser e.g. emitting 1064nm, the 100mW laser will look much brighter because the human eye can't see infrared wavelengths!