Why does it sound different when i listen to an iphone recording with versus without headphones?

I post little covers of myself singing and it just seems that without headphones, my voice sounds more resonant and full but when i plug in my headphones it just sounds really whiny and lacking bass. Is the sound with headphones a more accurate sound?

If the headphones are of good quality, they are giving you a sound which is closer to reality.

BUT: Cheap recording device have a poor recording and sound output quality anyway, so the ONLY TRUE and HONEST feedback comes from a fully trained vocal teacher after an OFFLINE one-on-one audition.

Then again, you are now hearing your voice completely outside of your skull for the first time, so it IS supposed to sound different, just because now you are hearing your voice WITHOUT any resonance from your skull bones and cavities.

Only an in-person analysis by a knowledgeable individual can resolve this.