The amount of connected sessions of PL/SQL Developer in Oracle depends on the "Session Mode" preference of PL/SQL Developer (one session in "Single session" mode, two sessions in "Dual session" mode and separate sessions for most of PL/SQL Developer windows and for object browser in "Multi session" mode).
If you are looking at v$sessions using PL/SQL Developer, then you'll always see at least one session active - it will be the session that PL/SQL Developer is using to retrieve the v$session data.
In "Multi session" mode, if you are querying the v$sessions using "SQL Window" of PL/SQL Developer (and not the "Sessions..." tool), then the memory use for that session will depend almost only on what queries you executed in that specific "SQL Window" before querying the v$sessions (and including that particular query too). As far as I know, in this mode, even triggering Code Assistant in that specific "SQL Window" will not use that window's session, so should not affect it's memory use.
Regardless of what I wrote above, you are referring to memory usage of Oracle server, not the PL/SQL Developer application, so if there's something wrong, then you should probably ask your Oracle DB server administrator for help in diagnosing if this level of memory use is something you should worry about or not, and if yes, then what may be causing that (including showing you what specific actions/queries cause that effect).