Clydesdales/Athenas (200+ lb / 91+ kg) - Does tire size help with broken spokes?

Reading Rivendell's catalogue, it seems that a heavier load (in the rider or loaded touring) needs a bigger tire. Could a larger tire (larger than 700cx23!) help with periodic broken spokes?

My friend is about 250lbs, very strong legs, and keeps breaking spokes on his Rush Hour. I've suggested larger tires (don't know how big that bike will accept). Or does he need to get better spokes and/or rims?

Larger tires at a lower inflation do provide a more compliant cushion. That said, it depends on whether they're breaking from the impact or from torque sheer. Clyde's can put a lot of foot pounds of torque out.

As an ex-Clyde, I'd say the rim would taco before the spokes should break. At least that's what happened to me. 250 lbs should be within tolerance of most bikes. I finally had my first spoke break a couple of weeks ago, and I don't think it's because of my weight. I switched them all out to 14 ga. double butted spokes, and hope to have no more trouble. There's no shocks on the bike and high psi in the tires, on a bumpy trail, so I think that's what caused the broken spoke.

Spoke tension will be reduced slightly by an inflated tire as it is compressing the rim to some degree so it should hold that a narrower high pressure tire will reduce your spoke tension more than a wider lower pressure tire.
Here is a short article on spoke tension:
http://www.cyclingnews.com/tech/fix/?id=tm_1

I am unaware if this reduction in spoke tension is significant enough to contribute to broken spokes in a properly built wheel.

I am not a physicist, but I don't think bigger tires would help except when you hit a bump. There is going to be the same amount of weight on the wheels, regardless of the size of the tires. In fact, there is going to be more weight on the wheels with bigger tires, because the tires weigh more. Additionally, the contact patch is going to be shorter and wider, with the wider tires. I would think this would focus the weight onto a smaller cross-section of the wheel. As Tom pointed out, tire size will have no effect on torque.
This seems to be intuitively wrong, but intuition and physics don't ever seem to get you to the same place.

I have never read the Bicycle Wheel, nor do I intend to, but I did find this claim in a much earlier post:

I have been reading Jobst Brandt's "The Bicycle Wheel." Amazingly, the effect of twisting the rear hub under acceleration -- even in a sprint -- makes a pretty small impact on the spokes. Brandt indicates that a fully-inflated tire produces a greater affect on the spokes than the cyclist's pedaling forces!

I have always been suspect of claims that the torque a rider puts out presents a problem with spokes. I think a hard application of a disc brake probably results in more stress on the hub/spokes than pedaling could.

so what would that mean regarding my original question?

Your original question:

Larger tire volume at lower pressure will likely reduce spoke breakage by some factor. I can't quantify the exact reduction in probability, but the softer ride is less stressful on the bend where they enter the hub.

I am not a physicist, but I don't think bigger tires would help except when you hit a bump. There is going to be the same amount of weight on the wheels, regardless of the size of the tires. In fact, there is going to be more weight on the wheels with bigger tires, because the tires weigh more. Additionally, the contact patch is going to be shorter and wider, with the wider tires. I would think this would focus the weight onto a smaller cross-section of the wheel. As Tom pointed out, tire size will have no effect on torque.
This seems to be intuitively wrong, but intuition and physics don't ever seem to get you to the same place.

Bumps, potholes, etc are the major source of stress on a wheel. If us clydes only rode on perfect tarmac, we wouldn't need very strong wheels at all. When we nail a pothole, the stress on the wheel is far higher than that sustained by just our body weight. If I could ride on perfect pavement, I'd be able to get away with 18 spoke weight weenie wheels, instead of 36 spoke 3X Deep Vs.

It would mean that the stress cycle of the spokes is increased by a higher pressure tire but I would doubt that it actually makes that much difference.

The bike you referenced (Rush Hour) appears to use an Alex DC19 rim. If that is the case then it is a pretty narrow rim and unlikely to take a very wide tire in the first place (rims have max tire widths, you could research it on Sheldon Browns site) so it is probably a moot point to suggest a bigger tire.
The rim itself is described as an economy rim, which does not necessarily mean it is weak but there are no doubt stronger ones out there. It is also listed as 32 spoke, all other things being equal a 36 spoke would be stronger but that would mean replacing the hub and rim as well.

If I am looking at the right bike it is a single speed with a 48/15 or 48/16 gear ratio so torque is certainly not going to be an issue as a teenager in the granny gear combo on his mountain bike will generate way more torque at the rear wheel.

If I were having the same issues I would take the wheel into a shop and have them relace it with new spokes. My favourite shop charges 35.00 for this.