Like others on the board (BlackLabel) I have done some work on stoneflies and their weightings.  A lot of folks equate a fly's size with its mass, thinking that big flies need to be heavy.  A bigger nymph fly, when tied light, tends to exhibit a side-to-side flutter that fish can't resist.  Other aspects of the fly (legs) tend to affect it hydrodynamically too, but a heavy fly glued to the stream bottom can't flutter much!

I first learned about the flutter from Joe Humphreys' videos.  He uses PT-style flies with prominent legs made from pheasant tail.  He mentioned the flutter effect.

(I may have said this before on the forum) This year I have done a lot of experimentation with selection of anchor flies with different weightings.  In one run I knew was full of fish, I experimented with three different weightings S, M and L.  The small (S) weight produced two fish, the medium produced six and the large only one.  I then went back to the most successful weighting (in this case M) and caught four more.  I have also done this on other occasions, usually on the Farmington, since I know it so well.

Try fishing with lighter flies to reap the rewards.  You will be amazed as once in a while even your light flies will still snag the bottom due to turbulent flow; this is proof that light flies still get in the feeding zones.  A light fly in a laminar (even) flow, will give you the longest possible drifts with a long leader setup.  My best drifts are over 50 feet.

Put another way...the mass of a fly used as an anchor is probably as important as it silhouette and colors.  I hope this is understandable, and sorry for making fly selection sound overly technical.