Fly goddess:

Larger fly lines have the capability to sink faster than lower weight fly lines. You'd think it would be the same since they are both round and made of the same material. However since they both have a core a smaller fly line has a greater percentage of core than a larger weight fly line. Thus the larger the line weight the greater the sink weight.

The other issue is that if you have a line that is tungeston coated there is only so small they can make it. Rio's smallest deep sink line is about as thick as backing but it's line weight is a 7 weight. It casts well on a 5 weight but it still scales out at a 7 weight for the first 30 feet.

John