Here's a useful comparison of the seat maps between the two types of internationally-configured 777s that UA operates:
Former UA (2-4-2):
http://www.united.com/web/en-US/cont...77/200/v2.aspx
Former CO (2-2-2):
http://www.united.com/web/en-US/cont...77/200/v5.aspx
The difference in personal space can clearly be seen, and this is what makes the biggest difference between the two in my experience. The seat size is very similar - both unequivocally have plenty of space for the average sized person to sleep.