I've been to WDW, DL, and DLP, and yes, they are very different...
DLP has 2 parks, Disneyland and the Walt Disney Studios. I've only been to Disneyland park, but what I saw I was very impressed with (jet lag messed me up and I hadn't slept in 72 hours and pretty much couldn't feel my legs
). Everything feels very new and clean compared to WDW and DL, and the cultural differences from the US make it a really unique, yet still familiar because it's Disney, experience.
Yes, DL is smaller than MK. Again, the Disneyland Resort has two parks, Disneyland and Disney's California Adventure. Disneyland has several different attractions and many "originals" - attractions that have been in the park since it opened in 1955. DCA is a work in progress (much like the Studios in Paris). It has several popular attractions (including Soarin' Over California and the Tower of Terror), and is receiving a major overhaul from WDI over the next few years.
Each resort has something unique and different to offer even a casual guest (not just obsessive people like me
); however, if you're looking for a resort where you can stay on property for 10 days and not run out of anything to do, you're only going to find that in WDW.
That is a
very basic comparison between the 3 resorts. I hope it gives you an idea, though.