What you have to remember is that the sun is about 3000-4000 miles above us. If you was watching a sun that was at ground level, alongside you, then it should shrink and grow as it moves towards you, or away from you. But since it is so far away from us, it has already shrunk in size, and we will hardly notice much of the size change as the sun moves toward the horizon.
Turns out p-brane has answered and explained the exact concern you bring up.
WHY THE SUN DOESN'T SHRINK TO A DOT on the FLAT EARTH CELESTIAL PERSPECTIVE EXPERIMENT SHOWS why
https://youtu.be/Vq5ixQytLXE
What you have to remember is that the sun is about 3000-4000 miles above us. If you was watching a sun that was at ground level, alongside you, then it should shrink and grow as it moves towards you, or away from you. But since it is so far away from us, it has already shrunk in size, and we will hardly notice much of the size change as the sun moves toward the horizon.