Crie o script mymv.pl
Perl:
#!/usr/bin/perl
use File::Copy;
$file = shift;
($name = $file) =~ s#.*/##;
if (-f $name and -s $file < -s $name) {
warn "$name not moved";
} else {
move($file, $name);
}
Em seguida, mude a linha para
find /tmp/dirtree -type f -exec perl mymv.pl {} \;