A standard Wii game disc holds up to or 8.5 GB (dual-layer) . A “highly compressed” Wii ISO is typically a .7z , .rar , or .wia file reduced to 100 MB – 2 GB using advanced algorithms (LZMA, LZMA2).
Here’s a concise informational text about — what they are, how they work, and important notes. Understanding Highly Compressed Wii ISO Files wii iso highly compressed