In a recent interview with Glixel, Destiny 2 World Director Steve Cotton revealed that the Io, Nessus and Titan zones will not be as large as the European Dead Zone. Stating..
I like Nessus the best. You’re not going to get to it until later in the campaign, but it’s probably my favorite. European Dead Zone is really good. It has more to do. It’s the biggest by far. It’s got lots of Lost Sectors, lots of dungeons, lots of just hidden little nooks and things to find. Nessus is going to take you into places a lot more – like what you were talking about with the Vault of Glass. They’re all very different.
This does make sense as the European Dead Zone was originally meant to be in the first Destiny game before it was cut.