That I can believe. At work we use the CAB Thesaurus for our keywords. It's about as thick as my Webster's unabridged, and it's still deficient in many areas (It's geared to the needs of the USDA). I haven't hefted the LOC thesaurus, but I suspect it's heftier.
I eschew thesauri.
And now I will go and be productive in a different orientation.