In most cases, the performance is determined by the number (and performance characteristics) of NAND chips in the SSD.
When comparing SSD's from the same product line, it is often the case, that they're using the same NAND chips in the SSD, but that the larger capacity ones have more chips than the smaller capacity ones.
In those cases, the larger capacity ones will often times have much better performance than the smaller capacity ones.
As an example look at this:
4KB Random read (QD32): Up to 98,000 IOPS (500GB/750GB/1TB); up to
97,000 IOPS; (250GB); up to 94,000 IOPS (120GB)
4KB Random write
(QD32): Up to 90,000 IOPS (500GB/750GB/1TB); up to 66,000 IOPS
(250GB); up to 35,000 IOPS (120GB)
which is taken from the cnet review of the Samsung 840 series of SSD's