I've no direct experience of Hampshire tups, although I know that a neighbouring smallholder uses one on their Llanwenogs too.
I posted to say that the external measurements of the ewe are only a part of the picture - pelvic width is very important, and as I understand it, Soays, Shetlands and Castlemilk Moorit have a wider pelvis than most commercial white sheep, which is one of the reasons why they seem to be able to lamb to a Texel no bother when some of their much larger cousins - Texels themselves, Mules, etc - can have problems.
Same thing applies to Jersey cattle - she's one of the smallest breeds but her pelvis is the widest, hence she can produce a double-muscled beef cross calf with ease, when some of her much larger Holstein cousins can struggle.
Another factor I've been observing in sheep is how much the mother puts into the growing lambs. I think the primitives and hill sheep are possibly better at not 'overcooking' their lambs, whereas some Mules will drain every atom of their being into producing mammoth lambs that they then are too **ackered to rear