The additional size comes from data alignment, i.e. the members are aligned to multiples 4 or 8 bytes.
Your compiler probably aligns int and pointers to multiples for 4 bytes and the double to multiples for 8 bytes.
If you move the double to a different position within the struct, you might be able to reduce the size of the struct from 24 to 20 bytes. But it depends on the compiler.
