The SSPI Embedded data files have some overhead added to the raw data to be sent to the FPGA. At the same time the SSPI Embedded code uses a basic compression technique to try to reduce the impact of the overhead. It is difficult to determine exactly how large the two files, when combined, will be.
A rough estimate for the maximum combined size of the SEA/SED can be performed by finding the bitstream size in the sysConfig technical note for the device family you are using. The maximum combined SEA/SED size is unlikely to be more than 10% greater.