Thanks, Roman. I started clean, but made many delete/import attempts until getting the import right.

Runned sqlite3_analyzer database.sqlite and got some interesting results:

*** Table ITEM_CHANGES and all its indices *******************************************
Percentage of total database......................  33.8%    
Number of entries................................. 4396      
Bytes of storage consumed......................... 3387113472
*** Table NOTES and all its indices *******************************************
Percentage of total database......................  11.7%    
Number of entries................................. 884       
Bytes of storage consumed......................... 1168904192
*** Table NOTES_FTS_SEGMENTS **************************************************
Percentage of total database......................   9.0%    
Number of entries................................. 153978    
Bytes of storage consumed......................... 899518464
*** Table NOTES_NORMALIZED and all its indices ********************************
Percentage of total database......................  11.7%    
Number of entries................................. 5304      
Bytes of storage consumed......................... 1168953344
*** Table REVISIONS and all its indices ***************************************
Percentage of total database......................  33.8%    
Number of entries................................. 5726      
Bytes of storage consumed......................... 3387277312

At glance, the bulk of the storage seems related to things that shouldn't concern my kind of usage (i.e. saving web pages). The question is now, how to slim ITEM_CHANGES, REVISIONS (and perhaps also NOTES_FTS_SEGMENTS) way down?