Yoshi's Story uses an LZ77-type compression format called SMSR00 which is very similar to SM64's MIO0 format. In the Yoshi's Story ROM, each SMSR00 block is preceded by a 16-byte 'CMPR' header which contains the size of the encoded block and its destination size.

struct CMPRHeader
{
    /*00*/ char cmprSignature[4]; // 'CMPR'
    /*04*/ u32  srcSize;          // size of encoded data
    /*08*/ u32  dstSize;          // size of decoded data
    /*0C*/ u8   _pad[4];          // structure padding
};

struct SMSRHeader
{
    /*00*/ char smsrSignature[6]; // 'SMSR00'
    /*06*/ u8   _pad[2];          // structure padding
    /*08*/ u32  outputSize;       // size of decoded data
    /*0C*/ u32  dataOffset;       // offset of the data section in the smsr00 block
};

• C: https://raw.githubusercontent.com/shygoo/ys-project/master/tools/src/smsr00.c
• JS: https://raw.githubusercontent.com/shygoo/ys-project/master/webtools/smsr00.js