>I feel sure though that most of the "giant races" myths come from later people, with diminished technical abilities, attempting to explain the existence of megalithic and other massive constructions.<
Yes, I agree. The Anglo-Saxons usually referred to Roman ruins 'as the work of the Giants' and I'm sure you're familiar with the following from <b>The Ruin</b> where an Anglo-Saxon poet looks upon the Roman ruins at (probably) Bath...
Well-wrought this wall: Wierds broke it.
The stronghold burst...
Snapped rooftrees, towers fallen,
the work of the Giants, the stonesmiths...*
* The Earliest English Poems. ISBN 0-14-044594-3. pp 1-3.