Disclaimer: game balance takes priority over lore and role playing.
that said if Essence was a Car then Empaths would be a race car driver and Sages, Magus, etc. would be more like Engineers
Empath are actually quite the blunt instrument in their understanding and use of Essence, they scoop it up and hurl it at people, they try to catch peoples Essence when they die and cram it back into their bodies (sometimes unsuccessfully) and they can use small scoops of it to achieve various other small effects to singular individuals. They do use Essence yes, but they don't have the profound and wide understanding of it, because they don't need it
Sage by design is supposed to be the most learned class with a wide understanding of all aspects of the world (as represented through their skills and as you noted, Essence pool) they may not be able wield Essence as bluntly as a Empath, but they have a much better understanding of it as a whole. Magus is the best of both worlds, they combine a wonderful understanding of Essence with a masterful ability to wield it, and this is reflected in its skills and Essence pool.
Sage = great understanding, poor wielding.
Empath = poor understanding, great wielding.
Magus = great understanding, great wielding.
The above is in no way cannon or legitimate simply my take and understanding of it.