Import an Inherited OLM Archive

I have inherited an OLM archive of 950 MB data. I think most of this data are emails and attachements thereof. What takes me wonder is, the widow told me the respective MS profile folder »Office 2011 Identies« had beeing contained 4.75 GB data. Does really making an OLM archive shrink the data volume at a fifth? Anyway, how can I import this data into DT3?

You cannot import an OLM file into DEVONthink.
You may want to seach for an OLM to EML converter or you could import the OML file into Outlook then export an MBOX file. You could import the MBOX file via DEVONthink’s File > Import > Files and Folders.