the guys with smaller herds are much more likely to feed the sows after weaning to put some condition back on them prior to selling.    They are more likely to have a place to do this and at times it pays quite well.

The big farrowing units don't have any place to put a dry sow to feed her with no intentions of breeding her again.

The spaces in the gestation building cost way too much to use a space for putting condition on a dry sow.      They are sold almost immediately after weaning.