The compression method in UPX is much simpler (therefore smaller and faster), but the last time I saw it was over 10 years ago, something may have changed. But LZMA should provide better compression.
I want to make the decoder small enough to be used in compressed executables. Where decompression performance doesn't matter because LZMA is far from fast decompression, and small code without inlining makes it worse (but bearable).
I have suggested them to look at my version, but I think UPX not much hardcore for that. Because I'm guessing decompression speed is more important to UPX than the extra 2kb.
I want to make the decoder small enough to be used in compressed executables. Where decompression performance doesn't matter because LZMA is far from fast decompression, and small code without inlining makes it worse (but bearable).