Warehouses have no consumption (the reduction of stock every 15 minutes), and I've not noticed any other type of decay over time for stock stored there.
The formula for city consumption has always been very closely guarded by the devs. What we have learned over time is that total population in the city and total number of people hauling a specific good are both included in that formula. This means that not only will each good have its own consumption rate (which could be drastically different from others in the city, and different from that same good in other cities), but also tells us that putting a single train hauling to the city will affect the consumption as much as putting a whole fleet of trains on it.
The only thing affected by total capacity (that I'm aware of) is the actual amount needed after consumption for the city to level. The dashed line is set to 2/3 of the city's capacity, and both amounts are shown. So the trick is to haul enough of the good so that the stock level remains on the right side of the line after consumption.
Obviously a dev might want to chime in on this, as I'm just an obsessed player who might've missed something