Sizes and necks, by format
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The reed diffuser bottle
The large end of the shelf, with enough height for reeds to stand and a neck sized for the throw you want. Neck diameter is the only intensity control left once the blend is fixed, so choose it on performance, not on looks.
The refill
Plain, larger where the shelf allows, and the format the customer pours from. It is also the one carrying the closure questions, so it deserves more attention than its price suggests.
Car and hanging diffusers
The small end, 7 to 15 ml, where the bottle is in motion and the seal during transport outranks everything else. These are the two formats where a leak happens in a bag rather than in a carton.
Why 20-400 dominates
Thirty-one of the 63 bottles carry it, which makes it the finish with the widest closure and reducer choice on this shelf. Standardizing a range on it is the cheapest decision available in this category.
Wider necks for reeds, narrower for refills
38-400 and 43-400 exist here for bottles that hold a bundle of reeds; 18-400 and 20-400 suit controlled pouring with a reducer. Matching the finish to the job rather than to the aesthetic is what keeps the product working.
Cost a bottle as a bottle
A 15 ml and a 120 ml cost close to the same to fill, label and ship, so a per-milliliter figure flatters the small one. At these volumes the packaging is a large share of the cost and the arithmetic is worth doing per format.
Decide the range before the first size
If the plan is a reed bottle, a refill and a car vial of the same blend, the finish question is answered once for all three. Choosing them separately produces three closures, three liners and three compatibility tests for no benefit a customer can see.