What comes out of the body are NOT "very very tiny" virus particles...those emissions are large drops of salt water with virus particles embedded, and masks "do" stop them.
Even when those saline droplets completely evaporate off all water, they are still far larger than individual virus particles.
I "misunderstood" nothing.
Here is a paper you can peruse if you want to learn a little
more about airborne virus particles and how they are studied.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2546863/