Well, maybe just for argument's sake I'll disagree and say I'm happy with the parks the sizes they are already.

I'll start with EPCOT, since I think it's an easier case to make. First off, that park is huge to begin with. It easily swallows crowds the other parks would have difficulty absorbing. And there is pre-planned room for additional pavilions to be added if there is need. Yesterland has a great article, focused on World Showcase, that shows how easily additional countries could be (and have been planned to be) added.

For the Magic Kingdom, I think the fact is that they don't NEED to expand it. As pointed out, it is the most popular theme park in the world. Even so, it rarely runs at capacity. Adding physical space and additional attractions would not necessarily draw significant additional guest counts. To me, it's like some 700hp supercar. Sure, the manufacturer could change the engine in the next model year to put out 750hp, but would anyone really care or be willing to pay a significant premium for it?

Added to that, increasing the square footage or the attraction count results in definite, concrete additional operating expenses. They have to hire more cast members, put in additional bathrooms, use more electricity, etc. So from a purely financial perspective (and as much as I hate to admit this, Disney is a business and needs to think of some things in profit and loss terms) it does not make sense to expand the Magic Kingdom. It DOES make sense to demolish old, unpopular attractions and replace them with new, crowd-drawing ones. After the initial capital outlay, their ongoing expenses remain fixed.