The main point is that the SSD consists of a bunch of memory chips, lets say 16 chips, and this effectively increases the throughput of the disk. Another factor is that larger area allows for better cooling during peak performance, and also the fact that disks used to come mainly in 3.5" and 2.5" form factors, so the manufacturers keep the 2.5" form for compatibility with other stuff. You can get SSDs in smaller packages but they will either cost a lot more or won´t be as fast as the normal ones.