The size of the genome (which can be said to be the amount of DNA) varies a great deal from one species to another. The smallest size, for viruses, varies from 2 thousand base-pairs to over a million.
Humans have around 3 billion - but some plants have more than 10 times as much.
There is no explanation for this wild variation in genome size.
This is linked to the so-called 'C Paradox' (where C is the number of chromosomes). There are huge variations in the number of chromosomes which species have. Humans, for example, have 46 (23 pairs) compared to 60 for chickens, or the Adder's Tongue fern which has around 1,200. These variations have not yet been successfully explained, but may have a relation to the age of the species - i.e. older species tend to have more.
Note that much of lifeform's DNA has been termed 'Junk DNA' - in some cases over 98%. The function of which (or non-function as the name suggests) is currently controversial. External Link
Also see: Genes