In many SSD the size of the NAND chips mean that in the larger SSDs there are a larger number of chips (and therefore more parallel read/write operations), than the smaller SSDs in the same range.
For example, in the Crucial M500 the best performance is in the 480GB and 960GB sizes, while the 240GB and 120GB are a little slower.
http://www.crucial.com/store/ProductMarketing_m500.aspx
Thanks a lot alex, it's so many help me before decided buying a ssd