Why is my usb mic quieter than my onboard mic?

At first listen, our ears would tell us that the on board mic is “louder” than your mic. But in actuality, this isn’t the case. The reason for this is “perceived loudness”.
When discussing the nature of human hearing, you find out that humans hear on a curve. That means that, within the frequency spectrum, humans perceive certain frequencies or “pitches” as being louder than others, even if they’re played at the exact same volume. These frequencies that we perceive as being “louder” are around the 1kHz to 6kHz frequency range.  (A team of scientists researched this phenomenon in the 1930’s.
So how does this relate to our microphones? Well, as you can imagine, the larger the diaphragm or “ear” of the microphone, the more it can “hear” or pick up.