Yup, the numbers are normal. The 40GB model have half the NAND chips and half the channels so write performance is impacted. However, I would not worry about it too much.
Basically, your disk usage breaks down into (order of importance):
Random reads - fine here since read performance is good
Random writes - fine here since small file write performance is good
Sequential read - fine here since read performance is good
Sequential writes - not very important since how often are you writing 100MB+ files to a 40GB SSD?