Limiting capacity of knapsack to a polynomial function of elements in the Knapsack problem

I saw somewhere that if we limit the capacity (weight) of the knapsack to a polynomial function of elements then the class of the problem changes to P, but it didn't say why. I can't figure out why is it true....

I think a found a solution but i'm not sure if it is completely right. The time complexity of the knapsack problem is equal to $O(W \cdot n)$, where $W$ is the capacity of the knapsack, so if we limit it to a polynomial function of $n$ then the complexity will change to $O(P(n) \cdot n)$ which is a polynomial complexity...

• So what's the question? It sounds like you have already answered your own question.... You say you can't figure out why it is true, but then you give a reason why it is true. What are your specific doubts or uncertainties about that reason? – D.W. Jan 30 '15 at 19:08

Since knapsack admits pseudo-polynomial time solution running in $O(nW)$, where $W$ is the bag size, if $W=O(n^c)$for some fixed $c$, then the pseudo-polynomial algorithm runs in $O(n^{c+1})$ which is polynomial in $n$.