The 18-year-old singer's well-known not just for her music, but for her habit of wearing baggy, oversized clothes to cover up ... and her first tour stop Monday night at Miami's American Airlines Arena was no different.

EXCEPT ... during a short video interlude between songs, Billie appeared on a screen in a dark tank top as she spoke to the crowd about the opinions people have of her.

Then, near the end of the vid, she took off the tank top to reveal just her bra ... and it was met with screams and cheers.

Here's what Eilish had to say in the video during the big reveal ... "You have opinions, about my opinions, about my music, about my clothes, about my body. Some people hate what I wear, some people praise it, some people use it to shame others, some people use it to shame me, but I feel you watching, always."

She added ... "The body I was born with, is it not what you wanted? If I wear what is comfortable, I am not a woman. If I shed the layers, I'm a slut. Though you’ve never seen my body, you still judge it, and judge me for it. Why? We make assumptions about people based on their size. We decide who they are. We decide what they’re worth. If I wear more. If I wear less. Who decides what that makes me? What that means? Is my value based only on your perception? Or is your opinion of me not my responsibility?"

Billie's famously avoided showing skin during her career because she's never wanted to be judged off of her body, but shortly before she turned 18 she revealed she wanted the option to flaunt her body and "look desirable." She said ... "I'm gonna be a woman. I want to show my body."

This isn't the first time Eilish has ditched the baggy wardrobe, though ... shortly after her 18th bday, she shared some bathing suit shots while on her Hawaiian vacation.