By giving small files and file tails (less than block size ends of longer than block size files) (command line)optional separate fragment collection paths Big-file tails do not influence the compressibility of concatenated small file fragments (as if default options where chosen) but big-file tail fragments still get concatenated with other tail fragments to potentially increase compression.
Cost: mksquashfs tool complexity
Gain: big-file tail fragment compression without decreasing small file fragment compression
e.g.: A, B, D are large files. C, E, F are small files
[Tail A]+[Tail B]+[Tail D]+[unused space]
[file C]+[file E]+[file F]+[unused space]