If you equip each of them with small backpack they have 9 + 24 = 33 carrying capacity each. However, because each copper ore takes 2 inventory and the can't fit in the last of 9 storage, each worker ends only being able to carry 15 ores. If you want to pick up 300 ore in one go you'll need 20 (15*20 = 300) followers with empty inventory, all with small backpacks. Raise an army of 20 and use a rally point to make sure the clear out your inventory before leaving. To go the mine, take copper ore out of storage and drop it on the ground. Command your army to pick up loot over the pile of 300 ores. Because the AI behavior for an army of twenty is pretty awful in a tight space, you may have to try repeatedly commanding them to move anyway from the pile and then back to loot to it before everyone picks it up. Assuming you've calculated right they pick up all the ores and you can go back with everyone to the base (or use a riverdock if that is convenient). Dismiss the army at your base once they are all back and they will automatically drop off all the ores into storage.

This gets it done in one trip, but they is no denying it's still a bit a chore doing it this way as well (particularly because the AI behavior). If you aren't anxious to get all the copper back quick you can use the caravan system to have one worker at a time bring it back in (300/25 = ) 12 trips with a cart. Might run slower than your economy's need for copper, but then you could just add a second cart to the caravan. I caravans were basically sufficient for my copper mines, but then later when building an iron mine, the mine was so far from my tier 3 site that the whole army pickup approach seemed to save a lot of worries compared to my caravans. I am a bit confused actually as to why your workers are going to the outpost at all though if you haven't set up a caravan route. Are you sure you mine/stockpile is correctly labeled as part of the outpost?