How can I prevent the revisions table cohesion_layout from growing too large?

Use the "Node Revision Delete" contributed module to manage your node and associated cohesion_layout revisions.

cohesion_layout revisions are deleted automatically when a parent node revision is deleted. To manage and regularly clean up node revisions automatically we recommend using the "Node Revision Delete" module. Regular deletions of node revisions done by the module will also clean up associated cohesion_layout revisions. 

For more information, see Managing revisions.