I feel it strange,because psp owns less ram,but still is able to load large arcade roms.
It's not impossible to run a game that takes up more space than you can fit in RAM--in fact, that's kind of the point of RAM, you're generally
not supposed to load a whole game into RAM, you read it off a cartridge or a disc or whatever else as you need it.
For emulation (particularly of ROM-based platforms as opposed to disc), this is a bit more complicated, because the original console expects to be able to access any data it wants
essentially instantly. A quick way to make this happen is just to throw the whole ROM into RAM, so any of it can be accessed at will, but as we see with Neo Geo ROMs, sometimes they're huge and it's just not possible to fit the entire ROM into RAM.
As such, you have to implement a paging system of some kind, which can copy the currently needed parts of the ROM to RAM while the rest sit on some slower medium (an SD card, on the 3DS). Then when the emulator asks for a section of ROM which you don't have in RAM, you have to empty out something else which isn't currently being used from RAM to fit the new section in, and so forth.
The downside to this approach is that it's slower than just reading it from RAM directly, because you're having to copy it into RAM
then read it back, all while the emulator is still waiting for the new data to be available to it can continue running. In the worst case scenario, you might have a game which is frequently reading from many different areas of the ROM, so you're constantly emptying out and replacing RAM and your game slows to a crawl because it's only running as fast as it can read data from the SD card.
Regardless, while this approach is certainly possible and works, it hasn't yet been implemented in RetroArch, and isn't something as simple as just saying "read from SD" instead of "read from RAM". It might happen at some point, but it also might not; it's a relatively small number of platforms which are so limited in RAM that they can't fit large game ROMs, and a relatively small number of ROMs which are large enough to need it. While the benefits of this approach are clear, it means somebody putting a lot of effort into a small edge case and working in the fairly boring area of file-reading, which probably isn't fun or rewarding for most hobbyist developers.
tl;dr It's kind of hard to solve this, might not work
quite as well as you'd hope, and would only improve things in a few specific scenarios. It's certainly possible that it could be done, but like everything, it takes someone with the time, interest and ability to implement it.