Fragmented MP4 (fMP4) is widely used for live streaming, but the ISO Base Media File Format (ISOBMFF) box structure imposes significant per-fragment overhead. Each box header requires 8 bytes (4-byte size + 4-byte type), and payload fields use fixed-width integers (u32/ u64). For low-latency streaming with single-frame fragments, this overhead can exceed the media payload itself. This document defines a compression scheme for ISO BMFF that replaces box headers with compact varint-encoded identifiers and sizes, and defines compressed variants of commonly used boxes with varint payload fields. The scheme reduces per-fragment overhead from ~100 bytes to ~20 bytes while preserving the full box hierarchy.