Due to a platform limitation that trigger on attachment won't be invoked when an attachment is sent via email I wrote a batch class to clone and parent email attachments from EmailMessage to a custom object Comm__c

global class BatchforOutboundEmailAttachments implements Database.Batchable<sObject>, Schedulable {    

    global Database.QueryLocator start(Database.BatchableContext bc){

        return Database.getQueryLocator([SELECT Name, Body, ParentID from Attachment Where Parent.Type = 'EmailMessage']);    
    }    

    global void execute(Database.BatchableContext bc, Attachment[] scope){

        set<id> parents = new set<id>();
        List<Attachment> newFiles = new List<Attachment>();
        Map<Id,EmailMessage> emailMap = new Map<Id,EmailMessage>();
        List<Attachment> oldFiles = new List<Attachment>();    

            for(Attachment att : scope){
                parents.add(att.ParentID);
            }

            if(!parents.isEmpty()){

       emailMap = new Map<ID, EmailMessage>([SELECT RelatedToId FROM EmailMessage WHERE Id in :Parents]); 

            for (Attachment att : scope) {
              if(emailMap.get(att.ParentId) != Null && emailMap.get(att.ParentId).RelatedToId != NULL){
                if (emailMap.get(att.ParentId).RelatedToId.getSObjectType() == Comm__c.SObjectType) {

                    Attachment newFile = att.clone();
                    newFile.ParentId = emailMap.get(att.ParentId).RelatedToId;
                    newFiles.add(newFile);
                    oldFiles.add(att);

                }
             }
            }     


       try{
            insert newFiles;
            delete oldFiles;
          }

           catch(DMLexception de){}    
        }

    }

    global void finish(Database.BatchableContext bc){
     }

    global void execute(SchedulableContext sc){

         ID jobId = Database.executeBatch(this,100); 

        }
}

In the start method I'm receiving 12k attachments which are not related to EmailMessage records where RelatedToID is Comm__c

How can I reduce the scope size so that less number of batches are processed?