I have x-checked that the F.mse_loss applied to Nx1xHxW tensors is such that the mean takes as normalization factor NxHxW: I have done a brute-force 3for-loops computation.
Now, it is generally states that learning on 64x64 patches would in general miss some details of larger images (ie 512x512).
But

this statement is only valid if the pixel resolution is increased between the two sets of images. Here, my 64x64 patches have rigorously the same resolution than the 512x512 ones, as the patches are originated from random cropping of 512x512 pixels images.
I would imagine the 512x512 loss (ie. the test loss) be worse that the 64x64 loss, which is not the case.

Any idea?