Why is dataset after tokenization far more larger than the orginal one ?
Hi ! This is because `.map` added the new column `input_ids` to the dataset, and so all the other columns were kept. Therefore the dataset size increased a lot.
 If you want to only keep the `input_ids` column, you can stash the other ones by specifying `remove_columns=["title", "text"]` in the arguments of `.map` Hi ! Thanks for your reply.

But since size of `input_ids` < size of `text`, I am wondering why
size of `input_ids` + `text` > 2x the size of `text`  🤔 Hard to tell... This is probably related to the way apache arrow compresses lists of integers, that may be different from the compression of strings. Thanks for your point. 😀, It might be answer.
Since this is hard to know, I'll close this issue.
But if somebody knows more details, please comment below ~ 😁
I tokenize wiki dataset by `map` and cache the results.
```
def tokenize_tfm(example):
    example['input_ids'] = hf_fast_tokenizer.convert_tokens_to_ids(hf_fast_tokenizer.tokenize(example['text']))
    return example
wiki = nlp.load_dataset('wikipedia', '20200501.en', cache_dir=cache_dir)['train']
wiki.map(tokenize_tfm, cache_file_name=cache_dir/"wikipedia/20200501.en/1.0.0/tokenized_wiki.arrow")
```
and when I see their size
```
ls -l --block-size=M
17460M  wikipedia-train.arrow
47511M  tokenized_wiki.arrow
```
The tokenized one is over 2x size of original one.
Is there something I did wrong ?