The official Samsung charger has a rated output of 2 Amps, which is quite a lot, hence its size.
On the other hand, standard USB has a 500 mA (0.5 Amps) maximum current rating, and an ordinary generic USB charger is likely to top out around there (or much less, if it's cheaply made).
The reason for the high current output of the official charger is the size of your tablet's battery - 7000 mAh. Even assuming 100% charging efficiency, it will take 14 hours to charge on the generic USB charger, while on the Samsung charger it will take a quarter of the time.
It's also worth noting there's not an established standard for USB chargers to indicate to the connected device that the device may draw more than 500 mA of charging current, and different manufacturers use different methods. To get fast charging, you'll either need an official Samsung charger or a third party product specifically designed to work with Samsung tablets.