Why are DC-DC ATX converters so big?
Even though it is DC/DC rather than AC/DC, all of the outputs are fully isolated from the input so 100% of the total energy has to go through transformers, the largest component type in the device.  To do that the input DC has to be chopped, just like the input AC in the other supply after it has been rectified and filtered.  So the only real savings in volume are the possible elimination or reduction of the large input capacitors.  The main switcher is low-volts-to-low-volts rather than high-volts-to-low-volts, but the total energy is the same and in round numbers the watts-per-cubic-inch is the same, thus the overall volume is the same.
And, because the input current is approx 10 times greater in the DC/DC supply, the transformer primary windings will be fatter, as will any noise suppression chokes.  One could argue that (for equivalent output power levels) the DC/DC supply should be larger than the AC/DC supply, not smaller
The supply is fully isolated because the manufacturer cannot know the &quot;quality&quot; of the DC source out in the field.  There are many reasons why you would not want the 12 Vdc input and the various DC outputs to share a common ground.  1500 V is the breakdown voltage rating of the insulation between the transformer primaries and secondaries.  It does not refer to any actual internal voltage level.