What is the difference between SIZE(n^k) and P/poly?
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($n^k$).