The larger card won't fit in the PSP. You can get adapters that allow the MS DUO to fit into normal MS slots. The small DUO card fits entirely inside the adapter. Meaning the smaller one is thinner and slightly narrower than the large version and the large one would simply not go in the slot on the PSP. If you look around on other forums, people have created adapters that allow a large MS to be attached to the PSP. This allows you to use the large MS that can go up to 4GB. Yes, there is a reason for this because you can load video and music from your computer onto the PSP and make it into a PMP. At the same time, if a person were to use this adapter for the larger card, the adapter tapes the large card to the back of the PSP and desn't leave a very fragile card sticking out the side.
I picked this up off of the Memorystick.com website:
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http://www.memorystick.com/en/ms/features2.html
Designed specially for use in ultra-compact and wearable devices for mobile
applications. Adopting higher-density, multi-stack technology, Memory Stick PRO Duo media is about one-third the volume and half the weight of standard-size media, but offers all the advanced functions of Memory Stick PRO media.
By attaching an adaptor, the media can be used with almost all Memory Stick PRO compatible products.
* Make sure to use an adaptor when using Memory Stick Duo media in products compatible with standard-size Memory Stick media. Without the use of the adaptor, it may become difficult to remove the media from the slot.
* Some products equipped with a slot compatible with both standard and Duo-size media do not require the adaptor.