It appears that Space Marines, and possibly future releases of Chaos Marines, might be getting slightly larger.
This is just in from a reader comparing the marines from Deathwatch, and a Tactical Marines legs.
via a reader on Faeit 212
It's not a very good photo, but on the left (unpainted) is the Deathwatch Marine's legs and on the right a Tactical Marine's legs from the latest kit. The hands and weapons are similarly larger, the hands being closer to Terminator sized.
I think that future releases, especially with the rumoured CSM release will feature these larger Marines. Assembled the difference is less obvious, partiality because the heads are the same size and the MK VIII's gorget.