The loadable segment in question appears to be the program's data segment.
The data segment in an program contains space for both initialized and uninitialized program variables. Values for initialized variables are stored in the program's executable. Uninitialized program variables do not need to stored anywhere; instead space is reserved for them in a special zero-sized section named ".bss".
The file size of an executable's data segment can thus be less than its in-memory size.
To illustrate:
/*
* Space for the intialized variable 'x' would be reserved the
* executable's ".data" section, along with its initial value.
*/
int x = 42;
/*
* Space for the uninitialized variable 'y' would be reserved in
* the ".bss" section; no file space would be allocated in the
* executable.
*/
int y;
On unix-like systems, the portion of the data segment mapped to the ".bss" section would be zero-filled at program load time.