The thickness of the bamboo, in and of itself, won't affect the stiffness. Almost all the work happening in the limb happens in the first 20% of the back and belly thickness (40% total). I used to thin bamboo to 1/8, but often leave it 1/4" or 3/16" at least. If you have nearly half the limb thickness on the belly lam, you are going to be fine. When the belly gets closer to 1/3 the total thickness I start to worry a bit.
Bamboo/ osage is a stiff combination to begin with. My guess is that it the fact it is SHORT and REFLEXED is doing it to you. Getting reflexed and R/D bows back to brace often feels incredibly hard.