Gware,
I have a similar situation here at my place.
I mostly find 3 prongs (even 20-30 year old roots) that are in the 8-10' tall range and have something like a 1/4 oz root or smaller.
My soil test showed ideal PH (5.4-5.7) but calcium levels in the 1450 range.
Also the land has fairly mature hardwood timber and the shade may be a bit too much in many areas for ideal seng growth speed & size.
I expect it is a combo of lower than ideal calcium levels and light levels that tend to produce smaller seng plants, smaller roots.
Something you might look for on your proprety or where ever you are planting, or while scouting a good planting place is rock and morning sun locations.
Even though most of my property produces smallish 3 prongs described above - when I get down to the creek bottom area there is a bluff there that faces due east.
Limestone bluff (200' or so high) and the bottom 1/3 of that bluff has some dirt that is just full of rock chips and leaf mulch from where all of that falls down and settles there.
It also faces due east and gets some really good morning sun, but then is shaded very nicely in the evenings.
The seng I find there is completely different from the seng I find in my average woods.
Taller plants with big broad leaves and huge roots & berry stems that are only 5-8 years old.
Here is a pic I took that shows the contrast in the roots.
Notice the big fat roots with short root neck (5-8 years old) below and to the left of that 20.00 bill.
They came off of that bluff area.
The other 4 roots to the right of that 20.00 bill (long root necks 20+ years old) came out of the deep shade woods with average woods soil (not rock chip filled).
If you want to grow big fat roots in less time, I think you need to find some mineral rich soil (heavy on calcium) that gets good morning sun.
Check your place out for rocky places, bluffs, rock outcroppings, etc and spots that get morning sun if possible.
Good Luck !
TNhunter