The companies I have seen dba administering SAP is not that small really, 5000+ employees. My boss for example, was DBA in one of largest petrol company in Spain and he used to administer SAP and several others.
Same with Oracle applications, people expect dba administer oracle financials, crm etc
Same with peoplesoft, jd edwards. The problem is there is no such position called ERP administrators (or there is but noone uses it here), the guy who's administering the database is gonna look after the databases these ERP runs on as well