Normally, deraillers are pantograph mechanisms. So if the guide and idler wheels are in a parallel plane to one cog, they should be parallel to all other cogs, unless somehow the cogs aren't parallel.

If the hanger bracket is bent, the guide/idler cage will be out of parallel to ALL the cogs, not just the small one.

I'd like to see a picture, but could it be that the pins and bushings, the ones that fix the derailler pantograph mechanism, are worn? Could you have a worn out derailler?

Alternatly, could your cassette have been assembled poorly, with the small cog or cog spacer not seated correctly so that the small cog is not parallel to the others.

I guess I'd put the bike in low gear (big rear cog) and spin the wheel backwards (so the chain and crank move). Do you see wobble in the small cog? Then spin the crank backwards (so that the freewheel clicks). If you still get no wobble, I'd investigate how "loose" the derailler pantograph is. A loose setup would be unusual, but if the cage is parallel with all other cogs but not the small one, and the small cog is parallel with all other cogs, then the derailler is not maintaining the proper constant angle it needs to.

How old is the derailller? What brand? If it's "Golden Happy Butterfly" brand from over the Western ocean, and the rest of the bike works great, I'd be tempted to try a new inexpensive derailller.
Alternately,