It is common to say "God bless you" to someone who has just sneezed. You are extending a wish for good health.
"May God bless you" is more formal. It is used to invoke the approval/support of God for a specifc action or purpose. "May God bless you for your profound kindness in helping the poor." May God bless you for changing my flat tire on this dark, dangerous street." "May God bless you and keep you safe during your tour of duty in the army."