SisQó -- Downsizes for 'Thong Song' ... and It's Just Sad

"She had dumps like a truck truck truck ... "

In the mood now? Good ...'cause we got SisQó performing his giant, monster, ENORMOUS hit, "Thong Song" last night -- which is HUGE -- but where he performed it is the real story here.

We found the R&B star doing his signature song from 2000 in Washington, DC suburb ... Alexandria, VA. But not in an arena -- a sports bar. A REALLY cramped, small sports bar.

You gotta see the video ... 'cause poor SisQó has to dodge bouncers and waitresses just to do a dance move.

Reminder ... "Thong Song" went all the way to #3 on the charts ... and SisQó used to perform it to sold out arenas with thousands of people singing along, "That thong thong thong thong thong!"

Things change.