Kylie Jenner posted a new makeup tutorial on Friday ... but there were a couple of reasons why many viewers were super distracted.

KJ shot the video in her car while talking about her beauty line and what she likes to mix and match ... but her massive boobs took center stage.

Of course ... she totally knew what she was doing with a low-cut black top -- and the way she was moving when pushing them together ... which was so obvious, it was borderline hilarious.

Last summer, KJ finally copped to her boob job, telling fans she got "445 cc, moderate profile, half under the muscle!!!!! silicone!!!" via Dr. Garth Fisher.

As for Kylie Cosmetics ... the beauty line has helped make Kylie a very wealthy woman ... who clearly never needs to worry about drowning.