"Shelter Deck" means that the scantlings (frames) stop one deck below the weather deck, and that first lower deck is open.
"Full Scantling" means that the scantlings go up to the weather deck. There is no visible difference, but that accounts for the weight difference. The dimensions are indeed the same.
The major difference in the types is that the "Full Scantling" ships can reach all cargo holds with their own cranes. The "Shelter Deck" types need external dock cranes to reach some holds.
Here are pictures of both:
http://drawings.usmaritimecommission.de/drawings_c1.htm
And the links to pictures of all US Maritime Commission types:
http://drawings.usmaritimecommission.de/drawingmain.htm