This article details it much more scientifically than I can.
Suffice to say, the struggle is real, babies. Out of character, I'm going to keep this short instead of being my usual verbose-self. But, for the TL;DR version: you hear your voice differently than we, the listeners, hear it. You hear your voice inside a muffled box (your skull!). We do not (duh).
SO. Making peace with this (I know, it's a tough pill to swallow), and then learning how to properly calibrate your own perception of how you're singing, and the sound/tone quality that that results in, is an important part of your vocal journey. Particularly in Head Voice (see my Range & Registers post for more info on Registers), we often perceive our voices as too light/weak, and therefore overcompensate to make the sound more "full" and "powerful." This is a seductive and disastrous trap. Working with a knowledgeable voice teacher who can help you determine when you're singing in justright, or when you might be overcompensating (resulting in a strained quality/unpleasant tone quality/pitch issues/etc), is invaluable.
Inquire within with more insight/help with this.