there is an ability, Harvest (Gold and Lumber) at the Special - Unit ability list. You can change the value of Data - Gold Capacity to 20. that way you can make peons carry home 20 gold, but it would also mean that 20 gold will be deducted from gold mines. i don't think there is a way to make peons carry home 20 gold and still take 1 gold from mines. unless it's in a way triggered.