Hardness amplification of PRFs by increasing key length

Question

I was reading the GGM construction for PRFs and wondering the relation between key length and hardness. GGM construction does not seem to yield any significant improvements. Are there any PRF constructions which take into account key length for increasing hardness? Alternatively, are there any constructions which transform a $(t, \epsilon)$ PRF of key length $k$ to something like $(t^2, \epsilon)$ PRF of key length $2k$ or $k^2$?