Viruses are in water droplets of various sizes. The viruses themselves can be quite “sticky” and will adhere to surfaces. So even a poofy mask will prevent a good number of particles from escaping. It will certainly prevent a trajectory of virus right at someone else’s face. That’s why they are encouraged as source control.
A poofy surgical mask can even protect a wearer from an occasional viral cloud as long as the viral particles are fairly dilute like they could be in well-ventilated grocery store. The only time you really need to have a good mask is in small indoor spaces with a lot of people. Even airplanes have such good ventilation that your only worry is people right next to you.
Here’s a thread from a while ago with lots of links. @Retzbu_Tox provided a ton of excellent links nearly half-way through the thread.