vacuumdb causes memory drain.

Hello Gurus,

Looking for some help, and if anybody has come across this type of scenario.

Running "vacuumdb -a -z -v" from the cron on one of our test environment, and
noticed that memory of the server goes down from 3.4GB free to 150MB. Once
the process is over, memory is not released, is it normal or something needs to be
changed ? also other processes start running slow afterwards due to low memory.

O/S: Ubuntu VERSION="16.04.4 LTS (Xenial Xerus)"
PG : PostgreSQL 9.5.12
CPUs: 2
Memory:4GB
Swap:4GB

max_connections = 10
shared_buffers = 1GB
effective_cache_size = 3GB
maintenance_work_mem = 256MB
min_wal_size = 2GB
max_wal_size = 4GB
checkpoint_completion_target = 0.9
wal_buffers = 16MB
default_statistics_target = 100
random_page_cost = 4
effective_io_concurrency = 2
max_worker_processes = 2
work_mem = 104857kB

Thanks,
Sushil...