I know the science of going to bigger diameters is right but I just can't get my head around not having smaller diameters with more suction
Think of it as a series of pipes with water. The blower provides a given amount of "head" (height, pressure) so if you have a 6" pipe it will allow more water than a 2" pipe .. with the same amount of head (pressure). If you take a 6" pipe and neck it down to a 2" pipe the absolute flow volume will be reduced but the flow rate will go up (less water moving faster). With airflow its about the same.
Vacuum cleaners provide more "pressure" than bigger dust collectors, you can think of them as similar to house water line pressure boosters - they move a lot of water (air) through a small pipe.. but not nearly as much as the big mainline pipe in the street - which moves more water but more slowly.
You're basically working on that trade off. The smaller pipe will move air faster, but moves less of it. The larger pipe moves more air but does it more slowly. So you have to find a balance in that tradeoff where you are moving the air fast enough to keep the chips afloat (and grab the dust as it comes out of the machine) but are moving enough volume of air that it encompasses the area where you are collecting the dust from.
You also have to keep the pipe small enough that the air speed is high enough to keep the material suspended so bigger is not always better... its just better right up until the point its not
The capturing material at the source point brings up the other challenge and also helps explain why _more_ air is sometimes better than _faster_ air. The pressure on the air coming into the hose diminishes as the square of the distance from the opening (yes there are tweaks to help/hinder that plus or minus.. working in generalities here please just play along
). You can observe the same behavior with a hose siphoning water out of a pond/tub, if you hold your hand right over the end of the hose there is a lot of pressure, if you move a little bit away from the opening the pressure drops substantially and increasingly so over distance. A bigger hose - and more volume at a slower rate - diffuses less at a given distance because the initial flow rate is higher. Again you can see this in water a small hose has great point application but a larger hose can move the whole pond in a hurry. A simple science experiment with a bucket, a couple of hoses of different sizes and a few drops of food coloring can make the diffusion distance very visually apparent. You can also see how you can get more pressure out of a smaller hose by raising the height of the bucket versus having the bucket lower with a larger hose - but the overall flow rate can still be higher with the bigger hose.
A contrary point to the more air is better than high speed air is when you have a very effective point collection. For instance the Festool routers have a very well designed dust shroud and since you are collecting from a very small space a lower flow but higher speed collection is more efficient there. For something like a table saw though you just can't get the collection system laid out to be that small very easily and for things like jointers and planers its pretty much impossible.
I've somewhat intentionally ignored friction loss thus far - the short version of that is that smaller hoses loose pressure much much more quickly due to friction loss than larger hoses (and corrugated much much quicker than smooth). So if you're trying to plumb the pipes for any distance the accumulated losses tend to consume the airflow capacity. Again water works the same way. Try a 1/4" hose and you'll never get very much flow through it no matter how high you raise the bucket and a 3/4" hose will have a lot more flow than a 1/2" hose from what you might naively expect the difference to be.