As stated already, Amazon S3 indeed requires Listing Keys Using the AWS SDK for .NET:
As buckets can contain a virtually unlimited number of keys, the complete results of a list query can be extremely large. To manage large result sets, Amazon S3 uses pagination to split them into multiple responses. Each list keys response returns a page of up to 1,000 keys with an indicator indicating if the response is truncated. You send a series of list keys requests until you have received all the keys.
The mentioned indicator is the NextMarker property from the ObjectsResponse Class - its usage is illustrated in the complete example Listing Keys Using the AWS SDK for .NET, with the relevant fragment being:
static AmazonS3 client;
client = Amazon.AWSClientFactory.CreateAmazonS3Client(
ListObjectsRequest request = new ListObjectsRequest();
request.BucketName = bucketName;
ListObjectsResponse response = client.ListObjects(request);
// Process response.
// If response is truncated, set the marker to get the next
// set of keys.
request.Marker = response.NextMarker;
request = null;
} while (request != null);