8/9 would be bigger.
This would be because, the number on the right side of the dash, is how many times 1 is split up, and the number on the left is the amount of split up chunks you would take.
In this case, 5/6, you would have 1 split into 6 chunks, and taking 5 out of 6 of them. 8/9 would be 1 split into 9 chunks, and taking 8 out of 9 of them.
Since there are more parts in 8/9 and you're taking almost all of them, it would be bigger then 5/6.
So therefore 8/9 is bigger.
