My favorite example is the meg's of commas and quotes that just aren't needed in memory.
Depending on the language and internal represenation, for any comma and quote not needed (which for a JSON document with, say, an object without nesting, is just 5 bytes per entry), you might get your strings doubled in size (because e.g. the language converted your fitting into 8bit utf-8 strings to its 16bit native string format), or have some huge constant boxing overhead (in say, Python), and several other fun things besides...
Depending on the language and internal represenation, for any comma and quote not needed (which for a JSON document with, say, an object without nesting, is just 5 bytes per entry), you might get your strings doubled in size (because e.g. the language converted your fitting into 8bit utf-8 strings to its 16bit native string format), or have some huge constant boxing overhead (in say, Python), and several other fun things besides...