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It's more than that. Several processes with small, independently garbage collected heaps are not as efficient as a single process with a large heap, parallel threads, and a modern concurrent GC (e.g. the JVM's ConcurrentMarkSweep GC)

In addition to that, processes severely inhibit the usefulness of in-process caches. Where threads would allow a single VM to have a large in-process cache, processes generally prevent such collaboration and mean you can only have multiple, duplicated, smaller in-process caches. (Yes, you could use SysV shared memory, but that's also fraught with issues)

The same goes for any type of service you would like to run inside a particular web server that could otherwise be shared among multiple threads.