There isn't any compression going on here. Your example:
is equivalent to:
echo super user
The echo command, like all shell commands, does not see the whole command line. Instead, it sees an array of arguments that are parsed for it by the shell. Like it says in bash(1), a command line argument is separated from the next argument by 1 or more spaces. So when the shell is parsing the command line, it pulls the arguments out, one by one, and passes them to echo. (We'll skip over the fact that echo is not a separate program, since conceptually it acts like one.) So echo sees a first argument "super" and a second argument "user" and doesn't even know how many speaces separate the two arguments. It prints them, one by one, with a single space separator. The other command:
exactly equivalent to
echo "super user"
passes one argument, and that argument has embedded spaces, which echo obediently prints out.