Your question is a bit unclear, so there are two possible answers:
what determines the portion of the moon that you see at night
That is called a "thin crescent moon." This phase occurs when the illuminated portion of the moon is only a small sliver, making it difficult to see in the sky.
when the moon waxes, the portion that we can see appears to get bigger, but it actually stays the same.
Gibbus moon
This is essentially correct. When part of the moon is in darkness, we can't see it. Additionally, the same side of the moon faces earth all the time as our satellite is locked in its orbit to constantly face earth. We only see the portion of the moon that is both illuminated by the sun and facing the earth.
what determines the portion of the moon that you see at night
when the moon waxes, the portion that we can see appears to get bigger, but it actually stays the same.
That is called a "thin crescent moon." This phase occurs when the illuminated portion of the moon is only a small sliver, making it difficult to see in the sky.
crescent
Gibbus moon
This is essentially correct. When part of the moon is in darkness, we can't see it. Additionally, the same side of the moon faces earth all the time as our satellite is locked in its orbit to constantly face earth. We only see the portion of the moon that is both illuminated by the sun and facing the earth.
The reflection of the light from the sun only hits a certain part of the moon, the unlit portion is nearly invisible from earth. The moon is always 50% illuminated. Our position keeps changing, and so does the portion of the lit-up half that we can see from where we are.
When you observe a crescent moon it is because only that crescent portion of the moon is illuminated by the sun from your particular perspective, here on Earth. If we were looking at the moon from a different angle in space, we would see different parts of the moon illuminated by the sun.
During a waxing crescent phase of the moon, the sun is illuminating only a portion of the side that faces Earth, creating the visible crescent shape. The unlit portion that faces Earth is not visible because there is no sunlight reflecting off of it to be seen from our perspective.
When you are seeing more of the lighted portion of the moon each night, the moon is waxing. When the lighted portion is diminishing, it is waning.
The moon is the moon. How much of the illuminated portion we can see from Earth is completely irrelevant to its mass.
The people who see a total eclipse are in the moon's umbra, the darkest part of the shadow where it completely blocks the sun. The moon is much smaller than Earth is, and therefore its shadow is as well. Therefore only a small portion of Earth can be in the moon's shadow at a time.