It is true that over time, the planets not only change their positions, relative to the stars, but they change in their appearance. They change in how bright appear and they change in how large they appear.
There are two factors that affect a planet’s size and brightness. For this discussion, we are ignoring the Earth’s atmosphere and particulate matter that is suspended in the atmosphere (clouds, dust, and pollution). The factors are: how far the planet is from Earth and how much of the lit surface of the planet is facing the Earth.
The further a planet is from Earth the smaller it will appear. Mars is a classic example of this fact. During August before last, Mars was as bright and large as it ever gets because it was as close to Earth as it ever gets. Since Mars is an outer planet, for all intents and purposes, it is always full, that is, we can see its fully lit surface.
One characteristic of an outer planet is, they do not go through phases, like the Moon. For this discussion, we will say that we can always see its fully lit surface. So for outer planets, size and brightness is determined by distance from Earth alone.
For the inner planets, the distance from Earth makes a difference but also the phase of the planet is important. As was just mentioned, the inner planets (Mercury and Venus) go through phases, as they travel around the Sun, relative to Earth. It stands to reason that the smaller the portion of the lit side of the planet we can see, the dimmer the planet will appear. The odd thing about the inner planets is, the less we can see of the lit surface of that planet, the closer it is to Earth. The more we can see of its lit surface, the further it is away from Earth.
If we consider these two conflicting pieces of information, the more of the lit surface we can see the further away it is and less of the lit surface we can see the closer it is, we find it hard to make a hard and fast rule about the planets’ brightness. Obviously, the further way the planet is from Earth, the smaller it will appear. To get the actual data about the planets, you can go to magazines such as Astronomy and Sky&Telescope.