Wendy never got very big on her first preg. A few weeks before birth her udder started to expand.
It could be that with the size of black belly sheep that they just don't get huge.
My group has yet to twin, so that may come into play as well. I cannot remember if Wendy's mom twinned or not.
You can't tell with my black belly sheep by looking at them. They hide it.
The udders do not get overly big either.