Hi, I can guess that it can be terms_in_memory. If it so, it's a trouble because I haven't got any answer how to manage this situation.
In 2.x the segments memory shown in more details:
"segments": {
"count": 8337,
"memory": "7.9gb",
"memory_in_bytes": 8570235911,
"terms_memory": "7.6gb",
"terms_memory_in_bytes": 8190950715,
"stored_fields_memory": "360.9mb",
"stored_fields_memory_in_bytes": 378518192,
"term_vectors_memory": "0b",
"term_vectors_memory_in_bytes": 0,
"norms_memory": "0b",
"norms_memory_in_bytes": 0,
"doc_values_memory": "749kb",
"doc_values_memory_in_bytes": 767004,
"index_writer_memory": "84.9mb",
"index_writer_memory_in_bytes": 89033244,
"index_writer_max_memory": "2gb",
"index_writer_max_memory_in_bytes": 2210971648,
"version_map_memory": "54.5mb",
"version_map_memory_in_bytes": 57193272,
"fixed_bit_set": "0b",
"fixed_bit_set_memory_in_bytes": 0
},
How many documents in your index and how many field is indexed?