Just as a comparison, general averages on each type of equipment (trying to be airline-neutral and preferring measurements that give one decimal point):

777 10-abreast: 17.2 inches https://runwaygirlnetwork.com/2014/1...o-matter-spin/
787 9-abreast: 17.2 inches http://www.runwaygirlnetwork.com/201...ying-on-a-787/
737: 17.2 inches http://www.airlinereporter.com/2010/...or-boeing-737/
A320: 17.7 inches http://www.airlinereporter.com/2010/...or-boeing-737/
A330/340: 17.8 inches http://www.seatguru.com/airlines/Cat...bus_A340_E.php
777 9-abreast: 18.5 inches http://www.seatguru.com/airlines/Cat...77-300ER_A.php
747: 17.5 inches http://www.seatguru.com/airlines/Cat...00_Class_4.php
A380: 18.5 inches http://www.seatguru.com/airlines/Qat...irbus_A380.php

From my experience in the back:
777 10-abreast: Very uncomfotable (above)
787 9-abreast: OK if not full. May be more uncomfortable if full
737: noticeably more cramped than an A320
A320: reasonably comfortable
A330/340: comfortable
777 9-abreast: very comfortable
747: minimum level I will accept for long-haul/if I have to sleep
A380: very comfortable (I think sxc's description is apt)