In astronomy, a trojan is a small celestial body (mostly asteroids) that shares the orbit of a larger one, remaining in a stable orbit approximately 60° ahead of or behind the main body near one of its Lagrangian points L4 and L5. Trojans can share the orbits of planets or of large moons.
Whether or not a system of star, planet, and trojan is stable depends on how large the perturbations are to which it is subject. If, for example, the planet is the mass of Earth, and there is also a Jupiter-mass object orbiting that star, the trojan's orbit would be much less stable than if the second planet had the mass of Pluto.
As a rule of thumb, the system is likely to be long-lived if m1 > 100m2 > 10,000m3 (in which m1, m2, and m3 are the masses of the star, planet, and trojan).
If I interpret that rule of thumb correctly, since the Sun has about 330,000 times the mass of Earth, a trojan system would be stable if the mass of the planet was less than 3,300 times the mass of the Earth and if the mass of the trojan object is less than 0.0001 the mass of the planet and lesss than 0.33 times the mass of Earth.
So it seems like the maximum possible mass of a trojan in Earth orbit would be about 0.0001 times the mass of Earth. If such an object had the same overall density as Earth, it would have only 0.0001 times the volume of Earth, and thus diameter of about 0.0465 that of Earth, or 592.506 kilometers, which is larger than any asteroid except for Ceres.
Since Jupiter, the largest planet, has 317.8 times the mass of Earth, the largest possible Jupiter Trojan would have 0.03178 times the mass of Earth. That is a bit larger than Ganymede, the most massive moon in the solar system, with 0.025 times the mass of Earth.
So according to that rule of thumb the planets in our solar system could have objects in their trojan orbits which are many, many times more massive than the most massive objects that are in their trojan orbits.
And the reason for that is probably statistical. For a planet to have an object in a trojan orbit two things have to happen.
The object has to form
The object's orbit has to eventually be modified so that it ends up in a trojan point relative to that planet.
And it is well known that there are only 32 known planetary mass objects in the solar system large enough that their gravity has pulled them into roughly spherical shapes.
They include 1 star, 8 planets, 5 dwarf planets (and a number of other candidates), and 19 moons. Others may be discovered in the future.
There are at least 198 other known smaller moons, plus at least 1 million known asteroids.
Asteroids vary greatly in size, from almost 1000 km for the largest down to rocks just 1 meter across. The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors, and are thought to be surviving protoplanets. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either battered planetesimals or fragments of larger bodies.
The number of asteroids decreases markedly with size. Although this generally follows a power law, there are 'bumps' at 5 km and 100 km, where more asteroids than expected from a logarithmic distribution are found.
So it is much more common for tiny objects a few kilometers across to form than for objects large enough to be the largest possible trojan objects for a planet to form.
In the early solar system there were many more large protoplanets. Gravitational perturbations between them caused most of them to fall into the Sun, or to collide with other protoplanets to form larger protoplanets, or to be ejected from the solar system and become rogue planets in interstellar space.
After the first few hundred million years or so most of the larger protoplanets were gone, one way or the other, leaving only the eight major planets and their moons in stable and widely separated orbits, and there was no longer any chance for large protoplanets to be captured into trojan orbits.
But tiny asteroids such a few kilometers wide were and are many, many, many, times as common as even the smallest spherical protoplanets, and the solar system still has hundreds of thousands of asteroids that size. So statistically it was much more probable for a planet to capture one or more of those tiny asteroids into a trojan orbit than to capture even one large rounded object into a trojan orbit.
I expect that star systems where the largest trojans are merely asteroid sized, as in our solar system, should be much more common than star systems where the largest trojans are planetary mass objects.
Astronomers have discovered so far fewer than 5,000 exoplanets in other star systems, out of the hundreds of billions of planets which should be in the Milky Way Galaxy. They have discovered several hundred star systems with multiple planets.
And they have found that many star systems are very different from ours. So it seems that it is hard to predict what the numbers, masses, and orbital characteristics of the planets in a star system will be. Random chance seems to have played a large role in planetary system formation.
So no doubt there are a small percentage of star systems where one or more planets have trojan objects which are as large as those planets could possibly have.