It seems like Houdini Digital Assets are imported with a meter scale. I like to model in centimeters, and cm is also the default UE4 scale. So it'd be nice if there was a way to change the import scale to cm.
EDIT: On second look, it actually seems to be importing at a scale of 1 Houdini Unit = 0.5 meters in UE4. So even when modeling in meters the HDAs are coming in at half the scale they should be.