I'm sure I'm not the only one would pay money for a tup that throws teeny tiny lambs that grow like stink
I've had too many vet interventions with monster lambs to wish that on any of my girls. (Not many, but any is too many.)
That's the general aim of good breeders
As far as I remember (without looking up the paper) optimum survival rates in commercial ewes are 3 to 5kg ---any lower and hypothermia risks rise, and higher and lambing risks rise dramatically
Obviously varies with ewe size & type
Ewe and lamb shape also has a large effect on lamb survival. Wedge shaped lamb with narrow shoulders that slip out easily----which is why the trend for 'nice square sheep' in some show circles is not a good thing
3 interventions a year out of 1000 ewes is my normal rate---usually ewe lambs or trips/quads. That's plenty