Is this an interview question? I find it a bit incomplete.
The best option will normally depend on what problem you are trying to resolve. Are you having issue will large memory consumption? Slow calculation performance?
1) Join merges two tables into one table. I would say this typically does not decrease data model size in a significant way, as the data as such is still the same.
2) Concatenate adds rows from one table to an other. This would result in the same total size, or in worst case larger size if the two table do not have exactly matching fields.
3) Link table introduces an additional logical table in the data model. In QlikView you are most often better of with a join instead.
4) Mapping table can be used efficiently to normalize your data values. If the incoming data is accurate it woudl have no effect.
In summary, all 4 options can be used to alter your data structure. The scenairo will detemrine what option gives you the result you are looking for.
