They are supposed to scale. If you log into the tale end of an instance that is about to die, you will notice that the population never quite seems to grow. Sometimes it's better to leave it, wait 15 minutes, and try to get in a different one.

Other times you will log into a fresh instance, and stuff will die in 5 seconds flat because it's scaled for under 20 players and a big crush of 40+ people hit it all at once.

It seems to take about 5-10 minutes for the adjustments to kick in.

It's entirely possible to solo stuff in there, albeit soul crushingly boring. I've killed the same two robots for hours trying to get For Absent Friends to spawn and gotten almost enough Bozja clusters to get the damn mount at this point.