Make firebird.conf empty and then add into it the single line:

StatementTimeout = 7200

Restart FB service.
Run some tool that can show memory consumprion per process, e.g. ProcessExplorer (on Windows).
Try following example (it will run infinitely so one need to cancel it via Ctrl-Break):

set list on;
set bail on;
shell del e:\temp\tmp4test.fdb 2>nul;
create database 'localhost:e:\temp\tmp4test.fdb' user sysdba password 'masterkey';

select *
from rdb$config where rdb$config_name = 'StatementTimeout';

set term ^;
execute block as
    declare i int = 0;
    declare res int;
begin
    while (1 = 1) do
    begin
        -- execute statement ( 'select rand() from rdb$database' ) -- [1] constant
        -- execute statement ( 'select ' || 1 || ' from rdb$database' ) -- [2] constant
        execute statement ( 'select ' || rand() || ' from rdb$database' ) -- [3] growth
        into res;
        i = i + 1;
    end
end
^
set term ;^

Watch into details for firebird process in ProcessExplorer (swich to 'performance graph').
You will see somth like:

No such problem if we comment out parameter 'StatementTimeout' in the firebird.conf.

Checked on 6.0.0.315.

PS.
I was unable to determine the exact value of the threshold of the number of unique expressions, after exceeding which memory growth begins. I checked MOD(i, N) function (instead of rand()) where N had different values.
It seemed firstly that N is ~16, but then i get results with constant memory consumption for greater values (i.e. for N = 17, 33, 67, 129 etc).