Because my LO's are skinny - I've never been able to rely on the weight guides for nappies.
Their waist measurements don't ever match up to the rise that I need.
Ittis are a particular example. DS always wore his ittis with the waist either overlapping completely to start with - or just meeting. They never got to the biggest waist settings. But I had to move him up for the rise, otherwise he'd widdle out the front, or poop out the back!
I think Eleanor will be the same. She's still got her small ittis overlapped - but is going to need a higher rise soon!
The larges were sooo much bigger than the mediums - so we went through a similar 'itti's don't fit' stage!
DS was well past 18months before the larges fitted him properly for good containment.
During that time we found our aplix nappies (blueberries, WNOS & tots bots worked better) - until he grew enough for the large ittis to fit properly!
have you tried fitted/ 2 part nappies?