DD1 was 3910, or 8lb10, no dystocia, I birthed on my back.

DS1 was 4410g, or 9lb13, his shoulders were stuck for a bit & required a bit of manipulation to get him out. I was on all fours.

DS2 was an emergency CS (for unrelated reasons). He was 4010g / 8lb13.

DD2 was a VBAC, she was 4710 / 10lb8, I was on my back, there was some manipulation but she didn't get stuck.

Not exactly the same as your situation (my DS1 didn't suffer any nerve damage, it was very mild dystocia) but thought I'd share my experience - every birth is different, and babies do not necessarily get bigger.

Scans for size are notoriously inaccurate, my sister was told she might need a CS because her baby was so big - she was 6lb 12 when she was born!

Good luck