I've never driven the CC, so I can't say from personal experience. But, here are the specs for each car:
Audi S7
Wheelbase: 114.7
Length: 195.6"
Width: 75.2"
Height: 55.9"
Front legroom: 41.3"
Front headroom: 36.9"
Front shoulder room: 57.2"
Rear legroom: 37.0"
Rear headroom: 36.6"
Rear shoulder room: 55.9"
Curb weight: 4,508 lbs
Horsepower: 450 bhp
VW CC
Wheelbase: 106.7
Length: 189.1"
Width: 73.0"
Height: 55.8"
Front legroom: 41.6"
Front headroom: 37.4"
Front shoulder room: 56.0"
Rear legroom: 41.6"
Rear headroom: 36.6"
Rear shoulder room: 56.0"
Curb weight: 3,851 lbs
Horsepower: 280 bhp
So, on paper at least, the S7 is noticeably bigger. Due to the cabin configuration, the cars might seem fairly close in size.
I do know that the S7 has quite a small rear seat - it's really more of a two-person car. The CC seems more capable of handling four (or five) persons.
The back seats of the S7 do fold down, which expands the trunk space from 18.9 cubic feet to 49.1 cubic feet. The CC has only 13.2 cubic feet in the trunk.
What you really get with the S7 is superior styling and performance.