Take a the SandForce SF2281 controller, it has eight channels to access the NAND array. Notice I say array, as an SSD is like several HDDs in RAID 0.
NAND is quite slow, for example 133MB/s per die, but if you can run the dies in parallel, and use all eight channels of the controller you get higher throughput. So let's say you have 16 NAND dies on a 240GB SandForce SF2281 based SSD, then you can make sure all 8 channels are utilised more or less all of the time. The data is stripped across several dies, just like it would be in an HDD RAID 0 array.
You also have several NAND dies stacked on a single NAND chip, and again you can use greater parallelism.
In smaller capacity SSDs you have less NAND chips, and perhaps less stacked NAND dies. You could still have the same performance as the higher capacity SSDs, but this would cost more, and the SSD would no longer be competitively priced.
It's a bit more complex than this, but in essence, that is why lower capacity SSD are generally slower than their higher capacity counterparts.
Different controllers have different sweet spots.
SandForce SF2281 is 240GB
Marvell is 256GB and 512GB
Samsung is 256GB (i think)
Everest and Everest 2 is 512GB, but by employing a rather clever FTL (Flash Translation Layer), the latest Vertex 4 for example is quite close between the capacities.