But the A380 was also 10 abreast. So like for like 3-4-3 on an A380 is allows for bigger more comfortable seats than 3-4-3 on a B777 (and I presume most other things), and the airline can shoehorn in a good lot of people!
But your missing the point.
The 777 was designed origionally as a 9 abreast aircraft and was marketed as such. However, the majority of the airlines started putting in slightly narrower seats and isles which allowed for 10 abreast. Even now, one of the major changes airlines want for the 777 in the future is having narrow fuselage walls so they can put the 10 abreast in with the safe comfort levels as the plane in 9 abreast.
The A380 is 11 abreast on the main deck when its in the full 840pax config. Emirates seat width on their current a380's, at 10 abreast, is only 18in in economy. Their 777's in 10 abreast is 17in wide. A 777 in 9 abreast has seats 18.5in wide.
The same is true for the 787. Boeing intended it to be 8 abreast but the 9 abreast option was for tour operators like Thomson or Thomas Cook, etc. But as it turned out, the majority of the regular airlines chose the 9 abreast and ironically, some tour operators are using 8 abreast.
So its not that the Aircraft is roomier, its just how the airlines have configured it.