Thermal mass acts similar to physical mass, except that it acts on heat.
Think of a physical mass for a moment. A large mass like a truck going downhill picks up speed more slowly than a smaller mass like a passenger vehicle. When it gets to the bottom of the hill, the truck will coast along the flat further than the vehicle will. When the bottom of the next hill is encountered, the truck will initiate the climb with more energy, but there is a point at which the vehicle will overtake the truck as it looses it's initial momentum.
Thermal mass acts the same way. You have the daily thermal cycle of heat absorption when the sun is shining, and heat release at night. You also have a seasonal cycle that acts on a much longer timescale.
The larger the thermal mass, the more "momentum" it has just like that truck. But it is a two way thing. You might get more of a buffer from the nighttime cold with a larger mass, but it will also take longer in the day to replace that lost energy. And in the fall a larger mass may extend your growing season a little, but in the spring, that mass will delay your growing season from initiating because it will suck the heat out of your greenhouse.
If you live in an area that gets overcast weather, the chances are that there won't be enough energy to charge a large thermal mass fully and so when the sunny days appear, the greenhouse remains cold, then the next weather cycle appears and your greenhouse ends up colder than the outside ambient temperature.
Unfortunately this idea that a large water tank painted black will transform a greenhouse simply doesn't work in the majority of real world situations without some sort of additional energy input. Geothermal is an option.
Compost is another although it tends to be problematic when it comes to decommissioning.
2
