I have long skinny kids and TBH I still find OSFM too chunky on Nate. Sized are much better, however the exceptions are the ones with snaps down the front, so itti tuttos, pop-ins, minilalas etc.
I love side snapping nappies as the can be adjusted for legs and belly.
Just got a Bubblebubs Candies nappy though, which is seriously good nappy. Holds more wee than a WW, but it probably would be a bit chunky for a 5kg bub.
Bubbamoo nappies are pretty trim and they fit my little guy well too.