Can speaker cables be too thick?

I would like to add my two cents, for anyone that's interested.
First, the impedance of the cable and amplifier combination (resistance plus inductance and capacitance) needs to be low relative to the speaker to maintain control - this establishes damping factor.
The resistance of the cable, which is a factor of the wire gauge, will affect the voltage/power of the signal to the speaker. The effect however, is constant for all frequencies, so it affects overall loudness but does not roll off the sound at either frequency extreme.
Next is the capacitance and inductance of the cable. In my experience, cable capacitance is not really a factor at audio frequencies, but inductance is. The inductance of a cable resists the flow of current through it, thus affecting the ability of the cable to pass high frequency transients and affecting the sound. Therefore, cables with lower series inductance will pass the musical signal more faithfully. If you look at Shadorne's link you'll see that it supports this.
I do listen to cables, and my experience has supported my conclusions.
It's unfortunate, but I think if more cable manufacturers published the electrical parameters of their products, more people would be able to associate the differences they hear with the electrical properties and much of what appears to be cable snakeoil would disappear.
This is just my opinion, based on my knowledge and experience, but it does drive the decisions I make when I select audio parts and components, and I am very satisfied with the results I've achieved. :o)

Just to confuse things further, I've found that changing power cords and related power-related "components" -- outlet/receptacles, conditioners, IEC/AC connectors, even fuses -- makes more difference in my system than speaker cables do. More than interconnects and substantially more than speaker cables. Two years ago I would have thought this to be COMPLETELY crazy. FWIW. Dave

Oh, I didn't know the copper conductor was the issue. When Dazdax mentions Cardas and MIT in the same line, I naturally thought he was talking about the visual size.

I do not have an opinion on thick cables. I will try that on my bass panels. I found very thin works wonders on the mids and highs.

Slv,

Yes inductance appears to be one of the factors that can begin to affect roll-off in highs. Skin effect can do this to (ever so slightly). However, the effects of inductance are still pretty small unless you go to large wire spacings (ribbon cables) and long lengths.

A difficult load (much lower than 8 Ohms) will lower the point at which inductance begins to affect high frequencies.

Flat - very thin- should be kept very short. That gives you the best worlds in mids and highs. Even 1 ohm benefits.