This can't be normal!

My son isn't sitting yet, but ad you say, when you hold then on a sitting position (which might happen sooner than you think!) it tightens on thei belly, so adjust the rise which may allow the wrap to stretch round further. When you say it's leaking, where is it leaking from? The legs? Or through the wrap?

We had to boost our little lambs with at least 2 boosters from about 4-5 weeks old. Motherease wraps were fine tho until we moved up a size. I didn't like thirsties duos but found the thirsties medium wraps or nature babies mediums (they have poppers so u can adjust the rise) were good.

Lola is in her BTP's and I have just opened them all out so they are on the biggest setting. The problem I have with her is that her tummy is MAHOOSIVE so she needs to wear them all harry high pants style or else there's massive gape at the tummy which causes leaks.

Its all about trying to see what works for your LO. I can see us having to snap them back down when she looses a bit of her lard

Every baby is different my dear! I've had three in cloth bums and though I've used motherease as our basic nappy with all of them, their different shapes and sizes meant we have had a journey through lots of different makes/sizes along the way! My third in particular seemed to out-pee his size 1 little lambs/totsbots very early, the girls were OK with them til much later. My middle child was chubby and outgrew her motherease onesize at the waist for several months til she slImmed through toddling, we needed size 3 totsbots and Toddle-Eases for a while.
So yes, it's normal for babies not to fit the "standard" size. Just try lots of different things til you get a good make/style/size.