I couldn't get our popper WNOS to fit Iris while she was still Nathan's age; I had to wait for her to grow a little. We used the same stuffing as you, except hemp instead of bamboo. I ended up selling a couple of ours on.
I don' t think its a problem with the stuffing, it sounds like a fit problem. I had probs with the fit of the OS on Katie for a while too - leg gapes and leaks. I ended up selling nearly all of them on to buy SSs instead. They fit her nicely now though and are probably one of the only front fastening nappies where we don't get leg gape and/or wing droop.