I think that the size of the collection and the rate at which it grows (number of new cards per day) influences the retention of older learned cards.
New cards interfere with retention of older cards → Retroactive Inhibition
Retroactive inhibition | psychology | Britannica
Learning theory - Stages, Acquisition, Retention | Britannica
This also applies in the opposite direction.
Older cards interfere with learning of new cards → Anterograde Inhibition
So I believe that collection size (Older cards) and Number of New Cards per day are variables that should be looked into.