I couldn't give you a probability but I would say that the likelihood would
definitely increase. Basically it can be said that each player is facing 5
outfield players when attacking goal, which is fewer people to get past. It's
kind of like two five-a-side games going on at the same time although players
are interchangeable.
If you exaggerated that either way and made it one-a-side in the outfield
you'd only have to get past one player to get an opportunity to score. Make it
100-a side and you've got to get past 100 players to get an opportunity to
score. I play 5-a-side weekly and on the occasions where we have 6 each it is
relatively low scoring and on weeks where there's 4-a-side it's a goalfest!
On top of that, you're going to get situations where one team has both balls
in its possession which would vastly improve your chances of scoring.
We used to play 2-ball football at school. If nothing else, it's mental!