Lets see if i can remember this right resistance is proportional to lenth and to radius squared, at constant fluid speed, and to speed squared at constant radius. Since cross sectional area incrase as radius squared then flow speed is inversely proportional to radius squared. This gives that resistance is proportional to radius to the fourth power at constant flow(not speed). Resistance is the sum of all resistances along the loop so changing radius of a part will have an effect even if it's not the smallest radius part. However since radius as sutch a huge effect on resistance the parts with the smallest radius tend to be the dominant parts.
As to greater volumes of water in a system causing greater resistance. Nope, it will take a bit longer to start going full speed but it will have no other effect.
Edward
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