Why do people say that bitcoin scales according to O(n^2)? In regards to bitcoin, Peter Todd says: "Fundamentally the tech itself does not scale. The whole idea of everyone knowing everything about every transaction, which is inherent to the trust model of bitcoin right now... that's simple: O(n^2) scaling. You just can't get away from that. No matter how much you push and pull that constant k value, you'll never get away from the fact that fundamentally it's O(n^2) scaling.

reference: https://youtu.be/Td6fwuI7F7U?t=4m59s

I assume n represents number of transactions?
What does k represent?

Also, is Peter referring to "Load" scalability? 
(wikipedia: The ability for a distributed system to easily expand and contract its resource pool to accommodate heavier or lighter loads or number of inputs. Alternatively, the ease with which a system or component can be modified, added, or removed, to accommodate changing load.)
n is the number of users. It is assumed that the number of transactions is Ө(n), and that the number of full nodes who must know all transactions is also Ө(n), so the total information (that needs to be propagated, verified and stored) is n*Ө(n) = Ө(n^2).

The assumption about the nodes requires clarification according to context, because there is no actual need for the number of nodes to scale with n.

k is the proportionality constant implicit in the Big Theta notation.