Yeah, I think scott_s is right, the issue is that Java just dumps pointers on the call stack to objects on the heap. And heap objects can end up all over the place in all kinds of non-optimal locations, like out of cache, or in different pages of virtual memory.
There's not really any such thing as just a block of a data allocated for however many bytes the object needs dumped on the stack...like good old C structs AFAIK.
It's one of the problems of languages that don't let you do your own memory management.
It's not a consequence of automatic memory management. Both C# and Go have automatic memory management but their memory usage would be roughly comparable to C++ in this scenario.
There's not really any such thing as just a block of a data allocated for however many bytes the object needs dumped on the stack...like good old C structs AFAIK.
It's one of the problems of languages that don't let you do your own memory management.