What is the difference between SIZE(n^k) and P/poly? What is the difference between $\text{SIZE}(n^k)$ and $\text{P}/\text{poly}$?
For reference:

$\text{SIZE}(n^k)$ is defined as the class of problems solvable with Boolean circuits (of fan-in two) with $O(n^k)$ gates.

$\text{P}/\text{poly}$ is defined as those problems over $\{0,1\}^*$ which can be solved by an infinite family of polynomial-size circuits ${C_n}$.


What is the difference between these classes?
SIZE($n^2$) consists only of problems that can be solved by circuit families of size at most $O(n^2)$.  P/poly contains problems that can be solved by circuit families of size at most $O(n^3)$, and those solved by families of size at most $O(n^4)$, and so on.
In particular, P/poly = $\cup_k$ SIZE($n^k$).