Okay, just thinking out loud really cos my LB girlies are growing fine (and my LB x GOS that I had last year were all 85Kg @ 6 months - but I didn't weigh in between)
I think it's been their slow start that's affecting them, perhaps the sow was not given additional feeding in the latter stages of preganacy and it's had a knock on effect ? Also if they're gilts they're going to be a bit smaller than Digby anyway - but to me, it does sound like they're not as big as they should be.
HTH
Karen x