I found OSFA to not suit DD at all and she has chunky thighs. Especially if they are snap ones because the lower rows of snaps roll out to rub on her inner legs. With velcro (like Green Kids) the problem is at the hips - tight enough on the waist means too tight around the leg. I am really happy with the fit of Ittis on her though.
what you describe, is how FRONT SNAPPING mcn fitted (or rather didn't fit) my DD.

i posted about that, got advised to try SIDE SNAPPING mcn, and that solved our fit problems.

Velcro closing nappies, once again are FRONT closing nappies.

the best solution for us, turned out to be sized, narrow crotched, side snapping nappies, with a few exceptions of osfm, narrow crotch, side snappers.

side snappers allow you to adjust the waist setting, independently of the leg setting.

my bub spent the majority of her nappy life, in one size of nappies, so we didn't have to buy heaps in each size.
(nb - prefolds and covers
from 6/7kgs - toilet trained at 11.6kg - in mediums e.g swaddlebees (5-11kg) and size 2's e.g wahm brands like dragonflymagik (7-15kg)
i had larges ready, but they barely got touched before tt-ing happened. Some NEVER got worn. i thought she would be in nappies until 3 at least.

so my initial thing of "must buy everythign OSFA" quickly changed. you just don't know until baby is there, what kind of body shape they will have and what will suit. so all those people who said "only buy one of each sort, until you know" - they were telling me the good oil.

and i was all set to go 100% aio - that changed too! I loved how quick the pockets and ai2's dried.

oh well, live n learn heh - just like the rest of parenting.