Some hardware accounted for the innate intensity of different hues; the SNES I believe used that principle to make some pretty cool looking blends and fades.
But yeah, if I recall correctly, if the relative intensity of pure white is 100% on an RGB display, then green accounts for about 20%, red is a little over 70%, and blue usually sits below 10%. In an additive color system these intensities appear to just add onto each other, which is why yellow is so freaking intense (it approaches 100%) and why FF00FF resembles a hot eyebleeding pink rather than an equal blend of red and blue, which it is; the intensity of the red simply dominates the blue.
As to why Yellow appears only about 2x brighter than blue like Indigo said rather than the 8-9x I described above, those numbers just describe the quantitave intensity. The actual visual intensity depends on your gamma settings and the sensitivity of your eyes, since the relationship between color attribute values and their visual intensity is quite nonlinear and subject to people's/an individual's sensitivity (green appears WAY more intense than blue because iirc the human eye is generally more sensitive to it)
But yeah, actually Pep and I were just rambling on about similar color phenomena the other night... it might be a good idea for all of us to pool our knowledge on the topic into something other people can use :p