As per my knowledge, there won't be any additional physical memory usage required by flume when running apart from while doing the installation and configuration of flume.
Flume is jut used as channel for data ingestion. Kafka does store the data into cluster whereas flume doesn't.
I assume your ceoncern is if Flume will take some space on local file system or not and not on HDFS.
Flume can have file channels where the data sits on the local file system as it gets consumed by sinks.
Also if you are using Spillable Memory Channel then data will be stored on local disk.
So we need to account for the file space required on local file system where flume channel is running.
But kafka or flume should not take space on HDFS. Even kafka log files are stored on local file system and not on the HDFS.