They also are for two different princesses. Cinderella in MK and Sleeping Beauty in DL.
I've always read that the Matterhorn was the visual mark of DL, instead of the castle anyway, so there's no need for a big castle.
When I was little, when we would drive to DL from San Diego, I remember that when we saw the first sight of the Matterhorn, then WE KNEW!!
Disneyland was right around the corner. My Mother would at one point say "Okay, start looking for the Matterhorn" and we would both shove ourselves over to one side of the car faces pressed up against the glass , like we were searching for Santa Claus. It was great.

The butterflies

in our stomachs would really start fluttering.
As far as the castle

goes, I always always felt like a princess walking through the gateway listening

to Jimminey Cricket singing "When you wish upon a star"
Walking through that part always seemed to slow my feet down.
As far as the size of the Castle, I am glad that it isnt visible from the freeway. I think it is a special jewel that we get to see when we are in the park. I think that Walt

made the size that he did because a. like Bumbershoot said, it is forced perspective. and b. because he hadnt made a castle before so, there was no other point of reference but what was in the plans.
Plus and this is just IMHO

,isn't World (I have never been there) just a massive place? So the Castle there should be bigger.
Just as an aside, I love these silly emoticons.

Especially that guy.