Hi,
While compiling, if you use the debug mode the compiler will add to the resulting file additional information that can help you when debugging. When U use Release option there is nothing. That exeplain the difference in the number of FF slices and slices
Number of Slices:
1) 1824
Number of Slice Flip Flop:
2)1422
Release mod
Number of Slices:
1) 1169
Number of Slice Flip Flop:
2)786