Small. Fast. Reliable.
Choose any three.
*** 96,98 ****
--- 96,100 ----
  
    SQLite (3.2.5)                 :   819,200 bytes
    Firebird (1.5.2.4731 embedded) : 8,736,768 bytes
+ -----
+ IIRC, as of 3.1 SQLite introduced an optimization that would result in the rows of a table consisting only of a single integer primary key column being stored entirely in the index (all SQLite tables have a "rowid" index).  That may account for the magnitude of the difference.