Homeschool science time. Everybody get a straw, and some paper to make spitballs.
Go on outside. Now, make a spitball. Put it in one end of the straw. Blow throw the straw with the same force as if you were speaking.
Mark how far it goes. Repeat 4 more times to get an average distance.
Repeat the process, but this time blow into it as if you were bellowing out "Holy, Holy, Holy."
Want to to guess which spitball traveled further?
And now you see the reason for restricting singing. I am very tempted to quote Jessie Pinkmann at this point, but will refrain.
While the opening of the straw is the same size in both instances, the opening of the mouth is not. Furthermore, unlike the spitball, what is expelled via the mouth/voice goes in 3 dimensions. So our home school experiment, which might be effective in demonstrating a general principle, overestimates the increase. The "model" doesn't accurately predict actual results.
How significantly?
I have, over the past few months has the opportunity to shoulder surf on a medical news aggregating service. Looking at the articles from journals like the Lancet, JAMA, NEJM, etc., there has been a great deal of information on aerosolizing and the difference that various activities make. Which is why the typical Church bathroom, with its small size and low power ventilation/filtration will pose problems. You may want to encourage people to flush with the lid down.