Sure, the virus is small, but it is carried in droplets, which are much larger.
OK, so the droplet gets caught on the mask. Meanwhile you are sucking air through that mask - how many viri do you suppose you suck out of that droplet? Or, say that droplet evaporates, then what?
Assuming that the mask does cover 100% of your nose and mouth, and the mask captures 100% of the droplets - what about your eyes? Your hands, that soda pop can, your water glass, your face. Scratch your head, eat a sandwhich. Spill some BBQ sauce on your pants, wipe it with your finger and lick it off.
Hey, if wearing a mask makes you feel better - wear the mask. Just wear it knowing that it’s as effective as wearing a sandwhich bag during a hurricane. You are still gonna get wet.