I've also found that wide crotched nappies are more prone to leaks around the legs on my narrow crotched guy, but mostly because the inserts 'bunch'. BumGs have always been a lot worse for me than GKs, which work brilliantly because the hourglass shaped inserts don't bunch. I also use GK inserts with BumG pockets with success. So perhaps it's a case of working out which shape inserts work best as well - maybe your GK shells will work well with BumG type inserts (ie: straight edged ones)? I think that the rule is that the insert needs to be as wide as the narrowest point in the crotch.

It does seem that narrow crotched nappies suit your DD best, and most OSFMs are wide crotched, so you may be better off with sized nappies. But that doesn't mean that your OSFMs can't be 'forced' to work for you, I've made mine work for me, it can just be a little trickier at times.

If your daughter is 'between' sizes on OSFMs then the bunching and leaks will be worse, I've always had huge problems with BumGs in particular whenever he moves up a snap setting. Because the shell was a bit looser than it should be, the inserts would bunch up and he'd just leak straight out of the side, and the insert itself would be nearly dry. But once they fit him better they don't leak. At these points a cover did help, because it prevents the inserts shifting around as much. So you may be experiencing an 'inbetween' stage at the moment, it might not last.

The other issue could be under-boosting. My 19 month DS has been known to wet through a d'lish in under an hour, lasts about two hours with a BumG with both microfibre inserts, and for going out or for a nap I add either a GK booster (3 layers of bamboo) or a narrow folded 4 layer bamboo booster. For nights I use either a dedicated night nappy, or a GK with the six layer hourglass insert plus two of the three layer boosters.

I also find that I don't get leg gape if I boost with a booster that is narrower than the original inserts, as the extra bulk is concentrated to the centre of the nappy, not the edges.

HTH!