Compression that only increases size of JARs is not usefull. These files cannot
be read faster (I/O) and there is some additional complexity when reading from
them. See output from 

> find nbbuild/netbeans/ -name "*.jar" -exec zipinfo -ht {} \;
Archive:  nbbuild/netbeans/ide8/modules/ext/jaxrpc16/FastInfoset.jar   557807   146
146 files, 527687 bytes uncompressed, 528417 bytes compressed:  -0.1%
Archive:  nbbuild/netbeans/ide8/modules/ext/jaxrpc16/jaxp-api.jar   192543   109
109 files, 172756 bytes uncompressed, 173301 bytes compressed:  -0.3%
Archive:  nbbuild/netbeans/ide8/modules/ext/jaxrpc16/jax-qname.jar   3361   3
3 files, 2890 bytes uncompressed, 2905 bytes compressed:  -0.5%
Archive:  nbbuild/netbeans/ide8/modules/ext/jaxrpc16/jaxrpc-api.jar   43840   52
52 files, 34614 bytes uncompressed, 34874 bytes compressed:  -0.8%
Archive:  nbbuild/netbeans/ide8/modules/ext/jaxrpc16/jaxrpc-impl.jar   5323171 
 1106
1106 files, 5094169 bytes uncompressed, 5099799 bytes compressed:  -0.1%
Archive:  nbbuild/netbeans/ide8/modules/ext/jaxrpc16/jaxrpc-spi.jar   27651   47
47 files, 18746 bytes uncompressed, 18981 bytes compressed:  -1.3%
Archive:  nbbuild/netbeans/ide8/modules/ext/jaxrpc16/jsr173_api.jar   49672   42
42 files, 42236 bytes uncompressed, 42446 bytes compressed:  -0.5%
Archive:  nbbuild/netbeans/ide8/modules/ext/jaxrpc16/mail.jar   270394   269
269 files, 464096 bytes uncompressed, 226574 bytes compressed:  51.2%
Archive:  nbbuild/netbeans/ide8/modules/ext/jaxrpc16/relaxngDatatype.jar   23497
  18
18 files, 19817 bytes uncompressed, 19907 bytes compressed:  -0.5%

Petr N. also found that recompressing of jaxrpc-impl.jar reduces it ssize
significantly.

BTW: NetBeans modules are shipped as uncompressed JARs (assuming that time spent
in decompression is longer than time to sequentially read longer content).