The Boeing 747 has an interior width of 20ft and the seating configuration is 3-4-3.
The Boeing 777 has an interior width of 19ft. 3in. and the seating configuration is 3-3-3, typically.
The difference in width of the cabins is so little, that Emirates and others have decided to cut back a little on the width of the seats and also the isles, and to add a 10th seat across.
Although it might look cramped, many who have flown Emirates claim to not have any complaints, as you do get the extra few inches in seat pitch.
Remember that the majority of passengers are not aviation enthusiasts and would not notice the difference between a 777 with 10 abreast seating and a 777 with 9 abreast seating.