In a way, this is plausible, as assigning a fresh string is a good situation to release memory back to the OS.

I'm not sure if pre-/resizing the buffer for a scalar still survives the various machinations. Obviously it breaks for sv_setpv, but I can see that breaking (fictional) code like the following:

    SV mysv = newSVpv("");
    SvGROW(mysv, 1024); // preallocate 1024 bytes as buffer for the re
+sponse
    sv_setpv("Hello World\0");
    some_systemcall_with_the_message_buffer( SvPV(mysv), 1024); // wri
+tes the response into mysv and expects 1024 bytes to be writable
[download]

... but I'm not sure if sv_setpv is supposed to copy the string or just copy the pointer to it.

If the threshold of 64 -> 16 bytes is sensible or if the resizing should only happen for larger decrements is to be seen.