Answer:
Larger pixels get more light during any given exposure, so are less noisy when viewed
Step-by-step explanation:
I presume you are referring to the size of the sensor for each pixel in a digital camera’s sensor. You’re not referring to pixels in a digital image e.g. a JPEG file, because there is no concept of “size” of a pixel in a digital image.
In sensors, if the size of each pixel’s sensor is larger, it gets more light and can average out the light it is receiving and thereby get a more accurate reading of the amount of light falling on it. With a smaller area, it’s reading is less accurate. Therefore, a dozen adjacent sensors in a smaller area, when exposed to exactly the same light amount, will give more widely divergent readings. If the sensors are larger and cover a larger area, then they will give readings closer to each other.
This variance in readings of adjacent sensors translates to what you’ll call a “noisier” image in layman terms. It will look more grainy. This seems to be a more serious issue when you’re shooting in low light.