1. The FX helmet *was* the only solution to very tall troopers, but RT now has an oversized bucket and there is another maker who will make correctly proportionate (e.g. ANH) buckets that are slightly larger next year, so there *are* now more accurate options and more on the way.
2. Even if you had a 100% accurate ANH bucket that is 10% too large for you, this is an issue. All costumes require that they be proportionate and fitted to the user, whether it's a Vader, Officer, or TK.