Typically when moms think storage capacity, they think breast size, the bigger your breasts the more milk, right?
Breast size does not necessarily dictate how much milk your body will produce and hold.
Breast capacity is how much milk your ducts and the space behind the nipple can hold between feeds.
Think of it like this, both you and a friend have cup, yours is slightly wider so holds a little more, you both place your cups under a magic tap that is dripping at the same rate and automatically switches off when your cup becomes full. Your friend needs to empty her cup more often in order to collect the most water while you can go a little while longer until you need to empty yours.
This is why some babys feed more often than others, it is not necessarily that your milk supply is low or inadequate, its more likely that you have a slightly smaller storage capacity and so your breast needs to be emptied more regularly for your body to produce more.
With exclusive pumping it is so important to not only stick to a schedule, but to read your body. Are you a mom that can happily go 4 hours between sessions, or do you need to pump more often in order to get the highest yield possible?
When milk is left in the breast too long, your body releases a protein called FIL (Feedback Inhibitor to Lactation), this protein builds up over time and stops the extra production of milk, so skipping that one pumping session a few days in a row or dropping sessions too early can damage your supply, likewise, when you top-up while breastfeeding, with every missed feed the FIL protein is building up and signaling your body to produce less milk, unless of course you are pumping during the top-up feed.
It is so important to remember that breastmilk production is based on demand and supply, the more milk you remove from the breast (especially during those all important first few weeks), the more milk your breasts will produce.
If you feel that you have a serious supply issue, speak to a board certified lactation consultant (remember to check credentials, you only want an IBCLC or SACLC) or one of the wonderful volunteer La Leche League Leaders in your area.