The Connies have 20 space left when finished with all of their components, so 10 additional points are added to the base structure of 30.
The 2273 refit has 4 space left over (although my FLGS owner and I believe that TPTB ignored the cost of the extra transporters or considered an emergency separation system to be worth 1 space instead of 2), and use that to bolster the size by 5. This is because the 2273 Connie has a lot more powerful and expensive equipment.
Moving over to the Galaxy, the problem is compounded even more. Even with 101 space available, the Galaxy goes over by 2 (even after sacrificing 5 space to wedge more stuff in). Again my FLGS owner and I subscribe to the idea that TPTB ignored a couple of point costs (add'l transporters, tractor beam), but the effect is the same. Relative to its size, the Galaxy has less structure.
But, comparing the two ships, is there any doubt the Galaxy would blow the Connie to smithereens, even with it's "weak" structure?
I think the car analogy hits it right on the head: a '56 T-Bird is going to physically survive a collision a lot better than, say, a modern Ford Windstar. However, the passengers inside the Windstar, because of things like airbags, crumple zones and better personal restraints, will have a better chance of survival (even though that equipment takes up more space).
Davy Jones
"Frightened? My dear, you are looking at a man who has laughed in the face of death, sneered at doom, and chuckled at catastrophe! I was petrified."
-- The Wizard of Oz