Whenever my son started leaking out of his diaper, I knew it was time to size up.
The Pampers available in the States (new "formula" Cruisers) have been causing loads of problems for children. There have been lots of complaints about chemical burns and whatnot. I haven't had any problems with them, apart from the new limited sizing. Pampers actually seem to run small for us. In Pampers, my 2.5 year old is practically busting out of a size 6 (yes, he's a giant) and when I manage to find some size 7s (which are nearly impossible to find), I buy them up.
I've since switched to a Huggies Extra Dry overnight diaper and haven't had any problems. Huggies, along with Luvs and some generic store brands, seem to run bigger than Pampers in general.