Why Scanning GSI on DynamoDb doesnt work as fast as expected when using CONTAINS?

Question

I'm new to DynamoDb. So if someone could explain to me, what am I doing wrong, or what I miss in my understanding that would be great. I'm trying to get the most effiicient way of searching for a row, that contains some value. 'm playing around with some test data to see how it works and how to design everything.

I have a table with about 1700 rows. Some rows have quite some data in them. There is PK - Id, And some other attributes like Name, Nationality, Description etc. I also added GSI on 'Name' With projection type 'KEYS_ONLY'

Now, my scenario is to find a person, that name contains given string. Let's say Name is 'Pablo Picasso', and I want to find any 'Picasso' My assumtion was, that if I am scanning the GSI it should be pretty fast, I understand, Scan can only go thorugh !mb of data, but I assumed, that My GSI looked something like this:

Name.	Id
A Hopper	2
Timoty c	3
Donald Duck	14

Having that in mind, I was sure it should find my row on first scan. Unfortunetaly my first scan went only through like 340 rows. I was able to find my row after 4 calls to Dynamo. When I made simillar scan, but not on the GSI it took 5 calls. which doesn't seem like that different.

Am I doing something wrong? Or do I missunderstood anything?

For testing purposes I'm using C# code like this:

var result = await _dynamoDb.ScanAsync(new ScanRequest(DynamoConstants.ArtistsTableName)
    {
        IndexName = "NameIndex",
        FilterExpression = "contains(#Name, :name)",
        ExpressionAttributeNames = new Dictionary<string, string>() { { "#Name", "name" } },
        ExpressionAttributeValues = new Dictionary<string, AttributeValue>()
            { { ":name", new AttributeValue("Picasso") } }
});

My index looks like this:

 var nameIndex = new GlobalSecondaryIndex
        {
            IndexName = "NameIndex",
            ProvisionedThroughput = new ProvisionedThroughput
            {
                ReadCapacityUnits = 5,
                WriteCapacityUnits = 5
            },
            Projection = new Projection { ProjectionType = "KEYS_ONLY" },
            KeySchema = new List<KeySchemaElement> {
                new() { AttributeName = "name", KeyType = "HASH"}
            }
        };

EDIT: I did some more digging and found out, that in fact GSI size is the same as the whole table.

...
        "TableSizeBytes": 5435537,
        "ItemCount": 1792,
        "TableArn": "arn:aws:dynamodb:ddblocal:000000000000:table/artists",
        "GlobalSecondaryIndexes": [
            {
                "IndexName": "NameIndex",
                "KeySchema": [
                    {
                        "AttributeName": "name",
                        "KeyType": "HASH"
                    }
                ],
                "Projection": {
                    "ProjectionType": "KEYS_ONLY"
                },
                "IndexStatus": "ACTIVE",
                "ProvisionedThroughput": {
                    "ReadCapacityUnits": 5,
                    "WriteCapacityUnits": 5
                },
                "IndexSizeBytes": 5435537,
                "ItemCount": 1792,
       .....

But why? Is there anything wrong with my Index creation?