I've stumbled over this today purely by accident. As it turns out the 'jwm-snapshot.tcz' extension offers a great "two-for-the-price-of-one deal" as it contains a 80 kB file in its root named '.tcz'.
Clearly such a hidden file is a bit tricky to spot, but it might help to explain why 'jwm-snapshot.tcz' is about twice the size of 'jwm.tcz' (whilst the 'Size' information in both .info files is anyway underreporting the true extension size).
Funnily enough the surplus file is itself a SquashFS archive (probably an older version of 'jwm-snapshot.tcz'), which contains another (albeit zero length) '.tcz' file in it's root.