I also heard that SD is used by smaller firms and when it comes to Bigger Firms CRM tool is used.
Its not like that..SAP CRM includes several modules that support key functional areas like Sales..
But SAP SD ( Sales and Distribution ) is important module of SAP and handles all the processes of order to delivery.
More difference can be found here.. Differences in the Organizational Data in SAP ECC (SD) and CRM - SAP Documentation
For more please search. " difference between sap crm and sd site:scn.sap.com "..