Amoeba has the largest genome in the world ever been observed, why? what is a biological significance of this enormous genome for that small Protozoan?

Why Amoeba ? 
The most common results for the search of the genome of Amoeba in google searching engine shows that Amoeba (Amoeba dubia) has the largest genome ever observed. 1,2,3 The result suggested that the genome size is 670Gb, which is incredibly large, considering that Human genome is 3Gb.

Anyone facing this result could ask a question- " why Amoeba? ", " Is there a decreasing trend in genome size from simpler organism to more developed ones? ", "Is there any upper limit that genome could have ? " and "Are these results are based on reliable source ? ", as I did.

Broad studies in the search of the largest genome showed that initial conclusions of the size of Amoeba dubia genome were based on biased experimental procedures.4,5  New studies, for now, showed that largest genome belongs to Plant Paris japonica (152.23 pg). Moreover, studies suggest that this genome size could be an upper limit which genome could take.