There are three things any one of which may limit the maximum braking
available; therefore for any given bike and load, the other two will be
irrelevant unless an improvement is made in the first one.

The first is inadequate braking power; the brakes simply cannot grip the
rim (or disc, or whatever) hard enough, and cannot do enough work. The
symptom here is that the lever is bottomed out or pulled as far as it can
be and nothing bad has happened as would be the case for problems 2 and 3.
This _can_ be alleviated with a lighter bike (but not with weight
distribution) since then there is less work to be done for a given
deceleration; it also can be alleviated by using both brakes, which
problems 2 and 3 cannot.

However it is extremely unlikely that this problem will be the significant
one unless the brakes are of inadequate design (which no modern brake of
reasonable quality is) or poorly adjusted (or rim brakes on steel rims in
the wet), or the total weight is really enormous.

The second issue is loss of front wheel traction, which is also the symptom.
Using the rear brake renders this impossible, but see the third problem
below. This one is pretty nasty, but fortunately is not ordinarily
possible (ie, some other problem will first manifest) with slick or
near-slick tyres when not cornering hard, even in the wet. I don't believe
bike weight matters here, within reason, because a heavier bike presses
harder on the tyre.

Therefore - if we don't have knobbly tyres, crappy brakes, or an enormous
load - we come to problem 3, where the forward weight transfer either
unloads the rear wheel until the rear brake locks it, or simply lifts the
rear wheel. _This_ problem is a function only of the angle described
between the CoG, the forward contact patch, and the ground (there's a
handwave here about going uphill or downhill), and as such changes to the
bike's weight or moving the CoG along a line passing through the front
contact patch do not matter.

In particular, a penny-farthing has a very high, forward CoG, and so this
problem will always be the limiting factor for braking. Conversely, a  
tandem is heavily loaded with a CoG well to the rear, and hence is more
likely to hit problem 1.

_But_ for a bike of normal design with a rider and load who may be heavy
but who are not exceptionally so, problem 2 (with knobbly tyres) or
problem 3 are always the limiting factor on braking if the brakes are not
actively defective, and that is why claims that disc brakes are more
"powerful" are met with scorn.