why declare arg as a array in FunctionCallInfoData structure

hi,

When I read the postgresql codes, I noticed that the FunctionCallInfoData 
structure(declared in the src/include/fmgr.h) contains two arrays 'arg' and 
'argnull'.
Why don't you declare it as a pointer and allocate the memory from heap? It 
saves more momery if 'arg' and 'argnull' declares as pointer type.

Can anyone explain it to me?

Thanks in advance.