An Xbox 360 ISO should typically be around 6GB to 7GB. If a "repack" is suspiciously small (like 500MB), it is likely a virus.
Look for the "Xbox 360 Redump" collections for the most reliable versions. Fight Night Round 4 Xbox 360 Iso REPACK
For those unfamiliar with Fight Night Round 4, here are some key features: An Xbox 360 ISO should typically be around 6GB to 7GB