If you are seeing half the magnitude (amplitude is the height of the signal, magnitude is the size of a complex value), that means you are using a 1/N normalized DFT (BTW, my preference). The sum will be the same (since the other half is orthogonal), you are just dividing it by twice as large of a number. You should also find that all the even numbered bins are zero and the odd bins have values. This is because the even bin sinusoids split evenly on your half way mark and the odd ones are integer and a half and therefore no longer orthogonal on those regions with your signal.
Hope this helps,
Ced