The whole point of a pooling layer is to reduce the h and w dimensions, so “same” padding would be sort of self-defeating. The other thing to realize is that the meaning of “Same” padding in general is not what you would naively expect: you only actually get the same output size in the case that stride = 1 and Pooling layers typically have stride > 1. For greater strides, even with a Conv layer (as opposed to a Pooling layer), you don’t get the same output size with “Same” padding. Here’s a thread which talks about that in a bit more detail.