Just noticed this when I was playing around with packing a skin file.
It seems the old version of the MXpacker file does actually compress the extensions/skins files. On the test that I did using the skins and an addon it shrunk the file by approx 40-50%.
IIRC your extension is one of the larger ones, so if you want to compress it then use the old version. Apart from that and it's command line based (although you can drag n drop) i'm not sure what else thee is different. But the files created still work.
Attached below as i'm unsure if it's still available anywhere else.
MXpacker_old.zip