From the Nintendophiles thread:
"If I read that article right, RS3 has 13 entirely new levels and 7 bonus levels NOT is RS2. Then, in addition, they included every single level and bonus level in RS2 in a coop mode. This is no doubt RS3 with an added RS2.1 bonus in it."
20 missions is not bad, not bad at all.

Especially when you take into account that all of RL's missions are included in co-op (that's like another 15, I think).