Before I start, I know its doubtful he thought this deep into it, but I wonder if being able to increase their capacity is an evolved succubi survival trait? Remember their biggest weakness. Without a master they basically have a hole in their mana pool, which when drained fully leads to death. If your pool is a bucket, thats gonna drain really fast. If your pool is olympic, that same hole is going to take awhile to drain down. So they constantly work to increase their potential mana pool so as to create a buffer of survival time should they lose a master for some reason. Thus giving them more time to find a replacement.

Further deep diving into the potential ramifications, someone who has a great lakes sized pool may have that massive capacity, but its doubtful their master can supply the energy needed to fill it quickly. Meaning said super succubi might be able to cast a TON of magic. But then after exhausting their supply, it could take a very long time to replenish. At least, i think dabbler said the master supplies mana as well as plugging the hole, I may be wrong. She may have also been meaning the regular treasure type O infusions the master is responsible for providing her is what keeps her mana full. But that could still be a problem for refilling the tanks if they can hold way more than normal.