South Europeans may be shorter but they are definitely not skinnier than the northern Europeans. The truth is that Letizia looked much lovelier when she had a few pounds on her before her marriage. There was a fresh look that has disappeared.
There are more small boned women in South Europe and small boned women tend to look skinny when they are thin. For example, Elena is also very thin for her height, but overall she doesn't look skinny since she has a bigger frame.
I do agree that Letzia looks better when she carries more weight, mostly IMO because she has an angular face and looks better when it's fuller.