This explains a lot! DS is only about 8.5kg so is in Crawlers. The last bulk box we used, the nappies just weren't up to scratch. They weren't absorbing as well as they had before and I was changing them every two hours. The plastic wrap the nappies came in seemed a bit different to normal as well.
I thought DS was just outgrowing them (crazy when they are meant to fit up to 11kg) so I have a box of Walkers to try. At the moment though, he is in Babylove Crawler which the girls at my mother's group love, and they're great. Considering how happy I am with Babylove I won't be buying Huggies again when we get through that last box.