“Simple filters” module offers a number of filters intensity rescaling, which can be used for compressing dynamic range of the image. Try RescaleIntensityFilter, IntensityWindowingFilter, ShiftScaleImageFilter, etc.
You can get the currently displayed window/level (or min/max) value from Volumes module, Display section.
Going from 16 bits to 8 bits is just 2x memory reduction, though, so it might not worth the trouble. If you increase spacing along each axis by 3x then you get 27x reduction.