Dataset file size on disk is very large with 3D Array
@lhoestq I changed the data structure so I have a  2D Array feature type instead of a 3D Array by grouping the two last dimensions ( a 224x672 2D Array instead of a 224x224x3 3D Array). The file size is now  223973964 bytes, nearly half the previous size! Which is around of what I would expect.
I found similar behavior in existing `datasets` collection, when comparing black and white vs color image, for example MNIST vs CIFAR. 
Hi, 

I have created my own dataset using the provided dataset loading script. It is an image dataset where images are stored as 3D Array with dtype=uint8. 

The actual size on disk is surprisingly large. It takes 520 MB. Here is some info from `dataset_info.json`. 

`{
    "description": "",
    "citation": "",
    "homepage": "",
    "license": "",
    "features": {
        "image": {
            "shape": [224, 224, 3],
            "dtype": "uint8",
            "id": null,
            "_type": "Array3D",
        }
    },
    "post_processed": null,
    "supervised_keys": null,
    "builder_name": "shot_type_image_dataset",
    "config_name": "default",
    "version": {
        "version_str": "0.0.0",
        "description": null,
        "major": 0,
        "minor": 0,
        "patch": 0,
    },
    "splits": {
        "train": {
            "name": "train",
            "num_bytes": 520803408,
            "num_examples": 1479,
            "dataset_name": "shot_type_image_dataset",
        }
    },
    "download_checksums": {
        "": {
            "num_bytes": 16940447118,
            "checksum": "5854035705efe08b0ed8f3cf3da7b4d29cba9055c2d2d702c79785350d72ee03",
        }
    },
    "download_size": 16940447118,
    "post_processing_size": null,
    "dataset_size": 520803408,
    "size_in_bytes": 17461250526,
}`

I have created the same dataset with tensorflow_dataset and it takes only 125MB on disk.

I am wondering, is it normal behavior ? I understand `Datasets` uses Arrow for serialization wheres tf uses TF Records.

This might be a problem for large dataset. 

Thanks for your help. 

Dataset file size on disk is very large with 3D Array 
 Hi, 

I have created my own dataset using the provided dataset loading script. It is an image dataset where images are stored as 3D Array with dtype=uint8. 

The actual size on disk is surprisingly large. It takes 520 MB. Here is some info from `dataset_info.json`. 

`{
    "description": "",
    "citation": "",
    "homepage": "",
    "license": "",
    "features": {
        "image": {
            "shape": [224, 224, 3],
            "dtype": "uint8",
            "id": null,
            "_type": "Array3D",
        }
    },
    "post_processed": null,
    "supervised_keys": null,
    "builder_name": "shot_type_image_dataset",
    "config_name": "default",
    "version": {
        "version_str": "0.0.0",
        "description": null,
        "major": 0,
        "minor": 0,
        "patch": 0,
    },
    "splits": {
        "train": {
            "name": "train",
            "num_bytes": 520803408,
            "num_examples": 1479,
            "dataset_name": "shot_type_image_dataset",
        }
    },
    "download_checksums": {
        "": {
            "num_bytes": 16940447118,
            "checksum": "5854035705efe08b0ed8f3cf3da7b4d29cba9055c2d2d702c79785350d72ee03",
        }
    },
    "download_size": 16940447118,
    "post_processing_size": null,
    "dataset_size": 520803408,
    "size_in_bytes": 17461250526,
}`

I have created the same dataset with tensorflow_dataset and it takes only 125MB on disk.

I am wondering, is it normal behavior ? I understand `Datasets` uses Arrow for serialization wheres tf uses TF Records.

This might be a problem for large dataset. 

Thanks for your help. 
 
 @lhoestq I changed the data structure so I have a  2D Array feature type instead of a 3D Array by grouping the two last dimensions ( a 224x672 2D Array instead of a 224x224x3 3D Array). The file size is now  223973964 bytes, nearly half the previous size! Which is around of what I would expect.
I found similar behavior in existing `datasets` collection, when comparing black and white vs color image, for example MNIST vs CIFAR. 