IIRC this is even a loading screen hint, but just in case anyone missed it (or is interested in the actual ratio), I decided to quickly test it.

Zonai Charges	Capsules	Ratio
1 small	1	1 : 1
2 small	3	1 : 1.5
3 small	5	1: 1.6
4 small	8	1 : 2
5 small	12	1 : 2.4
1 big	10	1 : 10
2 big	22	1 : 11
3 big	34	1 : 11.3
4 big	47	1 : 11.75
5 big	60	1 : 12

Conclusion: if you find a Zonai Device Dispenser with stuff you really like, feed it 5x Big Zonai Charges at once for maximum payout. The smaller Zonai Charges are best kept for other stuff, like checking what exactly a newly discovered dispenser spits out or refilling your battery in a pinch.

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EDIT / ADDITION:

If you mix-and-match, you get the following output in capsules:

Zonai Charges	0 small	1 small	2 small	3 small	4 small	5 small
0 big	0	1	3	5	8	12
1 big	10	12	14	17	21
2 big	22	24	27	31
3 big	34	37	40
4 big	47	50
5 big	60

The blank squares are impossible as you can only put 5 items at once into the dispenser.

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The Ratio:

1:X Ratio	0 small	1 small	2 small	3 small	4 small	5 small
0 big		1	1.5	1.6	2	2.4
1 big	10	6	4.6	4.25	4.2
2 big	11	8	6.75	6.2
3 big	11.3	9.25	8
4 big	11.75	10
5 big	12

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What about Construct Horns?

These are always a 1:1 deal. Even if you put 5x Captain Construct IV Horns into the Device Dispenser at once, will you get 5x capsules in return.