The XLR actually has less effective mass, as the contact points are just 2 pins, compare that with the big center pin of RCA.
It is true that I recommended using the cheapest, cheesy looking RadioShack Plastic RCA plugs on tone arm cables. One guy replaced the Cardas with the RS and found major improvement. "Music was dead and distant, now they are alive and vivid.".
The Eichmann's Bullet Plug takes this to the extreme by reducing the mass further even the outer RCA shell has been reduced to a pin.
The major benefit is that the inherant random noise in metal is proportional to the mass, the effect is defined as the Lentz law:
A current flowing through an inductor sets up a magnetic field so changes in the current result in changes in the magnetic field. Those in turn produce an induced voltage drop in the element, a voltage drop that opposes the change in the current. The magnitude of the induced voltage
is proportional to the time-rate-of-change of the current.
So basically, the ensuing mass of the cable following a weak signal such as the MC must be as light and approach 0 weight to be theoretically perfect. At 0.5mV, and to resolve a S/N of 60dB, which means we are looking at levels in the range of 0.005mV, or a noise in the nano-volt-sqr range.
Search the net for bio-medical research electrodes and the wires that use to probe bio-signal (very low level signal), the electrodes and wires are tiny!
Another part of this argument is that a balanced phono stage may not sound as good as single-ended because the noise and the doubling of circuit will quadrupled the noise.
The case of MC signal, preservation is the key, so I believe conventional wisdom of bigger is better must be put aside. The benefit of balanced is to cancel out common mode, but in MC, that's not the problem, rather it is the signal degradation.