The issue is that the psykers personal power really doesn't tend to be that much. Every psyker is also capable of using their own power to act as a conduit to the warp and using their own power to shape what they draw. The second method allows the psyker to wield vastly more power but runs the risk of drawing more power than they have the natural power to control.
I, personally, headcanon that the Imperium's measuring equipment is a method that aggregates the strength of the "Warp Shadow" of the Soul (responsible for Psykers being powerless in the vecinity of Necron Pylons, while still having personal power), the realspace psychic presence (which forms the bulk of most of the Primarchs' personal psychic power) and the strength of their value as a conduit to the Warp (which is where the Daemon shenanigans and non-Psyker Sorcerers come in).
The reason for Astropaths being a thing without them being planet-commanding monsters is that Astropaths are
generally low in personal "shadow" power, but high power as a Warp conduit, making their telepathy extremely efficient in
reach without offering much in the way of mind-control. This is also why they're the only category of sanctioned Psykers to
universally be Soul-Bound, because they're otherwise supremely corruptible, whereas Guard attachment duty is generally high in "shadow" power but limited as a conduit.
Of baseline humans, pretty much only Navigators are appreciable Psykers from realspace psychic presence, as it forms an important element of their Third Eye's functionality. It's also why
all Navigators eventually grotesquely mutate, no matter their self-discipline, as their Psyker abilities are overwhelmingly tied to their body, without the buffer of being in a "shadow" in the Warp to distance every little whim and belief from their bodies. On the upside, if they
bother to learn it, they're also ridiculously good Biomancers and Telekines, but instead tend to busy themselves on mastering "reading" telepathy to maximize their job skills.
Orks tend to to be high-"Bleedthrough", streamlining the process of psychic reinforcement of Nobs, Warbosses and technology immensely, while Eldar are high-"Shadow", having almost entirely self-contained Psychic power with very little function as conduits, offering a
considerable buffer against the pull of Slannesh. Meanwhile, the Tau, insofar as they
have psychic potential, are mostly "Conduit" psykers, without enough "Bleedthrough" or "Shadow" to be noticed or power anything themselves and scarcely enough "Conduit" function to be useful for Daemonic plagues, let alone actual possession.
The Tyrannids, despite what one may initially assume, are virtually
exclusively "Conduit" psykers, with the "Shadow in the Warp" being the soul-"Shadow" of their gestalt, a pseudo-Daemon that's as utterly outside context in nature as possible. And also actually stupidly weak, given the amount of biomass generating it, because the lack of significant "Shadow" or "Bleedthrough" psychic potential means it has
very little to draw on. And it requires specialized organisms with limited throughput to apply, anyways, so it isn't really
worth increasing its available energy, because they can't
do anything with it unless they make a full-on Hive Ship dedicated to channeling it.
The primary exceptions being the Biovore and Genestealers, of which the former is primarily a "Bleedthrough" psyker to accelerate their metabolism for minaturizing the everliving hell out of their function of Spore Mine generation, to limited success due to how little "conduit" function they have and lack of space to borrow the Exocrine's solution (the Exocrine shares the problem due to issues with internal self-damage from heat and overly-caustic substances, the "weapon" biomorph is mostly directing the stream), and the Genestealers, who
do have a use in maximizing the available energy of their gestalt in that it improves their function as a beacon, so they're optimized as "Shadow" Psykers, as well as adopting the properties of the host species. So Ork Genestealers are actually remarkably
terrible beacons. Generally, Tyrannid and Ork don't mix well in the slightest, their gestalts are
completely contrary in form.
Much more specifically, the Norn Queens, like the Biovore, are primarily "Bleedthrough" psykers for biomancy purposes, using their sheer bulk to still fit enough "Conduit" neurological structures to source the necessary power from the Hive Mind gestalt. This biomancy, like the Biovore, is used for rapid and compact gestation, in addition to being used for sculpting genetic contents and expression. However, unlike the Biovore, this is a highly conscious action, necessitating levels of independent action that the Hive Mind intensely prefers to stay away from.
The Hive Tyrants are a combat organism of the same general design, modifying genetic expression and setting Psychic energy from the gestalt into bioforms, whereas the Swarmlord is the ultimate expression of the Tyranid's potential for crafting "Shadow" potential within their paradigm, an existence unique within each galactic strain. And duplicating/fragmenting when the highest-level Tyranid organizational structure falls out of communication between its extremes; there's more than one mass of Tyranids in the universe, and they are entirely able to develop in
wildly different directions if separated far enough. But each one has a
single Swarmlord, which is always a seven-limbed abomination of close-range siegebreaking and tactical mastery.
