Eager global ordinals & doc values memory consumption

Hello,
just started using the Eager global ordinals option on my mapping along with doc_values (for reducing memory consumption) . Though still not loading any field data as expected, I notice a major increase in segment.memory_in_bytes. Does ordinals setting keep segments in memory for faster ordinal/segments updates or something?

Any help on this would be appreciated.

Oren

segments: {
count: 1577
memory_in_bytes: 12222041370
index_writer_memory_in_bytes: 12371428
index_writer_max_memory_in_bytes: 5404296015
version_map_memory_in_bytes: 9874984
fixed_bit_set_memory_in_bytes: 0
}