As of yet, a Process instance can't free memory.
The built-in callback has a self-reference to the intance not cleared after instance destruction.
I've tried to change both buildCallback and https://github.com/symfony/process/blob/v2.7.6/Process.php#L1373 usage to avoid self-referencing, but it can't be done without breaking BC, and the memory issue is still there.

A simple test script (that obiously can't be embedded in unit-tests)

$echo = function($inc){
    echo (memory_get_usage(true) / 1000000) . ' MB ' . $inc . PHP_EOL;
};
for ($inc = 0; $inc < 10000; $inc++) {
    $process = new Symfony\Component\Process\Process('echo 1');
    $process->mustRun();
    $process->stop();
    unset($process);

    $echo($inc);
}

Before:

1.572864 MB 0
5.505024 MB 9999

After:

1.572864 MB 0
1.572864 MB 9999

Ok, in this simple scenario 4 MB of RAM is nothing, but in our business app full of IOC where unset-ting is hard, our RAM explodes.

After stop(), the callback is no longer necessary.