In terms of the first example, a Shield of Spirit is very different from a shield formed from Air to protect a channeler from physical harm. They aren't the same thing, and don't have the same effect. The main difference is that defeating another channelers' weave involves you using your superior skill or power to weaken it. Thus, while a Shield of Spirit is weakened when another channeler uses his or her own connection or Shield to push back against it, this doesn't mean that using a weave to create a physical effect and having this physical effect meet physical resistance results in the weave being blunted - the weakening of a Shield occurs because two energy matrixes are affecting each other; it isn't the same thing as a physical force contacting another. Using a Shield against a physical object, for example, does absolutely nothing, because a Shield has no physical effect. If that were the case, Rand would constantly need to reinforce his sword of Fire to battle opponents due to how he was using it - he never had to and it always remained super-sharp and super durable until he untied it, because the flows weren't affected by physical contact. This is also why Siuan made the point about how pointless making a sword of Air would be - it was not that this weave would be less useful than a sword; in truth it would be much more useful and more powerful since it is forged of the same energy force that drives the Wheel of Time. But if you can make a sword of Air, why wouldn't you just create a barrier of Air or restrain the opponent using Air? The One Power is ultimately creative, and its' use is to create effects, that can be used for a diverse set of situations whereas a weapon is very limited and one-dimensional.

The strength of a channeler primarily seems to effect the area they can affect at one time with a minimum of effort. For example, the Windfinder Jorin, can work the weather in a far greater area and with more skill than most Aes Sedai (even more noteworthy is the commentary from the Forsaken that indicates they are stronger at working the weather than even Age of Lgends Aes Sedai working in a group could) - further, the Windfinders in general could use much thicker flows of Air than even Elayne could, and Elayne is much stronger than the one she saw. It turns out that Elayne and most Aes Sedai were so used to discrete uses of the Power, that they never practiced making vast, sweeping weaves. Despite Elayne's much greater strength in the One Power, the Windfinder could work the weather at a much greater distance and with much thicker flows of Air. The key difference was that using the Power like this seems to have taken the Windfinder more time than it would have taken a stronger channeler, and tired her out more quickly. Another example, is that Egwenes' strength in Earth allows her to convert a large object to cuendillar in a moments' time, while a weaker channeler with some strength in Earth could definitely do the same, but it would require much more time.

Basically, the strength of a channeler doesn't necessarily equate to bigger, better weaves. A channeler of average strength could erect a formidable barrier of Air given the time to erect it, but a stronger channeler could do the same with less time. Remember Moiraines' comment to Rand - Callandor, for him, would enable him to flatten the walls of a city with one blow, but for a less powerful channeler, the implication is that the city walls could still be flattened, but it would take multiple weaves (more time) to do so.

The One Power is a metaphysical energy source and the rules governing the physical world don't really apply to it. Making a more powerful weave often depends on your ability to control the energy inherent in a flow, and it is often the case that a channeler without specific Talents and strength is completely unable to form certain energy matrixes while others can form the matrix to make the weave work, but not to a degree to be useful, such as channelers unable to open a gate large enough to walk through but with sufficient strength to make the weave itself work, so they can make small gateways. or the example of how male channelers can lift things higher than female channelers with Air but almost all female channelers can make longer bridges of Air than any male channelers despite the strength difference. Thus a weaker channeler might need to use smaller weaves to build an effect, but the effect would be the same, albeit more vulnerable to a stronger channeler untying the flows than an effect erected by a more powerful channeler.

Also, this also ties into why I believe RJ said that men and women are basically equal in effectiveness with the One Power; while men generally have more strength in the Power and can channel more of it without getting tired or destroying their physical bodies, women generally have more dexterity, and thus can channel saidar faster and with less effort than men can do so with saidin. Thus at the end of the day they both end up completing similar tasks, assuming all Talents and flow strengths are equal, at the same time.