Complex sentence: Treatment was usually harsher on large plantations, which were often managed by overseers and owned by absentee slaveholders; in contrast with small slave-owning families, where the closer relationship between the owners and slaves sometimes resulted in a more humane environment.
Split into simpler sentences: Treatment was usually harsher on large plantations, which were often managed by overseers and owned by absentee slaveholders. By contrast, small slave-owning families had closer relationship between the owners and slaves that sometimes resulted in a more humane environment.