Lucene - trying to find out size occupied by each field in apache lucene index

Question

I am using java lucene 6.3.0

My document has 50 fields and indexing 500k documents takes 1gb of space. I am trying to recognize which field is taking more space in the index.

I have already tried analyzing the index using luke but the problem is it shows Term count of each field which need not decide the actual space taken by the field.

Is there any other option in luke which I'm missing?