This actually used to be the case in an earlier version of Factorio. It's mentioned in my changelog, that I don't know when exactly Factorio changed things, but somewhere between 0.14 and 0.16, they removed this ability.
They used to work exactly like that; all the extra materials would simply be pushed into the next slot over. Now, it doesn't matter how many slots a recycler has; the extra ingredients will simply be lost if you attempt to recycle over the stack limit.
And even this is a somewhat recent change from Factorio, as previously, the machine would simply "clog" partway through its task, becoming unable to recycle anything until it was mined and replaced (returning the clogged item back to you)
And even even then, this is on top of Another change they made to furnace logic, preventing the use of alternative output slots. Meaning, if you put in a belt, you'll get some iron and gears in slots 1 and 2. But if you then put in a green circuit, without removing those items, it will simply idle until they are removed. Previously, it would continue working until all output slots were filled with materials.
During this time, I made no changes to the logic of the recycler, and I have no control over how it processes materials. It's just a fancy looking furnace with a big list of recipes. Any changes to its logic would have to be done on the dev side of things.
If I wanted to "hack" it to act like that now, I'd have to completely ignore the usual "crafting" algorithm used by the game, and implement my own version, which would probably be inefficient and cause a lot of script lag. It works well enough the way it is now imo.
