It's entirely possible. When you substitute various nut meals and flours for white flour the amount (volume) often changes. I know with nut meals, for the best results, you are only supposed to substitute about 1/4 cup out of every 1 cup of flour. I have heard of lots of people who have had good results using more though. I think with soy flour it's a 1/3 cup replacement for every 1 cup of white flour. If your cookies came out nice and light that's good. They sound very cake like and those are really delicious in my book.
I found a nice soy website with lot's of info about it:
Soyfoods Association of North America - health benefits info and advocacy for soy products
Keep on baking Larry!!!
