Do size comparison after gzip

[michaeleiselsc]

For anyone thinking about these formats for client-server communication, they will want to know what the size of them is after gzipping or applying other forms of compression. It can be very surprising - JSON can beat binary formats.

[sburton84]

This can be especially important for Cap'n Proto, as empty or default fields remain in the data as all zeros, which will then be reduced when packing or compressing. If the packing feature of Cap'n Proto is not being used then it is important to use compression to reduce the size of the output.