Volume vs. surface area. The life more familiar to us is concentrated to what amounts to a 2D surface. Oceans are mostly devoid of life excluding surface waters where sunlight is available and seabed ecosystems.
One of the first people to consider this was Thomas Gold (https://en.wikipedia.org/wiki/Thomas_Gold), for example http://www.pnas.org/content/89/13/6045.
There's a nice review here (http://www.pnas.org/content/114/27/6895) but it doesn't have much detail.
I know the emphasis should be "some", but even then this is a lot of biomatter. How do they come to that number?