All, below is my code. While trying to make callout to an external API, I`m getting an error “System.CalloutException: Exceeded max size limit of 6000000” synchronously. I believe this is happening since payload that is being received is very large. I was wondering if there is a way to work across these limits. I tried using @future too however its giving me an error too "System.LimitException: Apex heap size too large: 12649268". Can someone please suggest a better way to get data in Salesforce?
 

HttpRequest obj = new HttpRequest();
    HttpResponse res = new HttpResponse();
    Http http = new Http();
    String reqBody = '{ "user": "test_user", "passwd": "2gbwefgnaiognawgona12" }';
    obj.setMethod('POST');
    obj.setHeader('Content-Type','application/json');
    obj.setEndPoint('https://test.samplepoint.com/api/UserSrvs.svc/Login');
    obj.setBody(reqBody);
    obj.getheader('Auth-Token');
    res = http.send(obj);
                            system.debug('Oauth Response: '+res.getbody());
                           system.debug('Header Auth-Token Response: '+res.getHeader('Auth-Token'));

            authtoken objAuthenticationInfo = (authtoken)JSON.deserialize(res.getbody(), authtoken.class);
            System.debug('objAuthenticationInfo: '+objAuthenticationInfo);


      Http h1 = new Http();
    HttpRequest req1 = new HttpRequest();
    String reqBody2 = '{ "Accountype" : "workforce"}';
    req1.setHeader('Auth-Token', res.getHeader('Auth-Token'));
    req1.setHeader('Content-Type','application/json');
    req1.setMethod('POST');
     req1.setBody(reqBody2);

    req1.setEndpoint('https://test.samplepoint.com/api/accservices.svc/accountfeed');//URL will be your Salesforce REST API end point where you will do POST,PUT,DELETE orGET
    system.debug('======req1========'+req1);
    HttpResponse res1 = h1.send(req1);
    system.debug('==========res1============'+res1.getBody());