My experiences mirror some of the others. Down at the shop we have two different braided materials that we use for loops. One is camo and the other is black. The camo stretches like you would not believe. The black stretches but not anywhere near as much.

When I initially tie the loop on I end up leaving just enough room between the two knots so that I can slide a nock in there. Then I " wiggle" a phillips in between the loop and the nock (turned sideways) and pull to get the " stretch" started. What I usually end up with is a relatively short loop if it is done in this fashion.