Why an empty matrix is 208 bytes?

In his book, Hadley Wickam explains why an empty vector is 40 bytes in the section memory. But what about a matrix? A matrix also has, at least, two integers in its attributes to store the number of columns and the number of rows. But it is not sufficient to explain why an empty matrix is 208 bytes. I was expecting something like 48 bytes.

> object.size(matrix())
208 bytes