Sometimes the full moon looks so huge. What atmospheric conditions cause that ?

Atmospheric conditions have nothing to do with it. There is a lensing effect low on the horizon but it actually makes things look smaller, not larger. The moon appearing to be bigger is just a trick of the mind. The mind percieves the sky near the horizon as being farther away than the sky overhead. As such, your mind subconciously thinks the moon near the horizon is further away, but because it appears to be the same apparant size as it usually does, the mind believes it must be bigger to appear that size, so you get this sense that it is larger than normal.