Why is my cohesion_layout_revision table so large?

Cohesion layout revisions are created at the same time as node revisions, but not always deleted at the same time.

Cohesion Layout revisions are being created and deleted whenever a node revision is created or deleted. This means that for each revision of the node with Layout Canvas there will be a Cohesion Layout Revision entity. This is normal behaviour for revisionable content entities in Drupal, however in specific scenarios it can become problematic if regularly updated nodes have a long lifespan (Homepage Node, for example).

There is an unresolved issue in Drupal Core about entity reference field items not being removed when the referenced entity is deleted. This applies to Cohesion Layout Revision entities. This means that if a node is deleted, the deletion process removes the node and its revisions, but not composite entities referenced by the node. In the case of Site Studio, this means that Cohesion Layout Revisions for deleted nodes are being left in the database as orphaned entities.
For more information, see Managing revisions.