When the database file is locked for physical copy, it should not change. But test case below shows that it grows.

isql -term !
create database 't.fdb'!
create table t (n integer)!
create procedure sp_load
as
  declare n integer = 0;
begin
  while (n < 50000)
  do
  begin
    insert into t values (:n);
    n = n + 1;
  end
end!

nbackup -L t.fdb

The file sizes are:
t.fdb 692224
t.fdb.delta 12288

isql t.fdb
execute procedure sp_load;

Now the sizes are:
t.fdb 3391488
t.fdb.delta 2551808