"in the movie the Telmarine army doesn't seem that big"
Only by modern standards, the size of the forces in Prince Caspian actually looked plausable for a quasi ancient/medieval style battle and was less `CGI horde` than most recent films.
In reality the massive LOTR style armies seen in recent fantasy films would be impossible to control by the `word of mouth` communications they would have had to rely on unless you assume the `Dark Lord` is using some sort of mind control over all his troops.
(If you think of it, thats why the good guys win in LOTR.)