When documents are stored in Tamino, the documents themselves are stored in data, whereas the indices are stored in the index space. By applying default native mapping with indexing means all terminal nodes are indexed. So depending on the complexity of the document it is not really possible to determine the the “explosion” of the document in terms of index and data.
An easy way to find out though: create a db, write a small java program say which writes 1000 ‘average’ documents. Then use Tamino manager to judge the average “explosion” factor. This is what we do sometimes to see what size of database we need to create but it only acts as a rough guide.
As a very very rough guide and this is my experience: data usage is 1 - 1.5 times the actual document size. Index is 1 - 2 times the document size. This applies to v2.3 of Tamino only. 
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