Ha! I give a new information: the loss depends on the cropping size, if I choose 64x64 for the test as it is for the train, then

The test loss = train loss (nb. the PSNR is a bit lower as I have adapted a little the definition so the max-min range is defined with the pixel values not the dtype values)
Is there a way to get the loss independent of the size of the image?
I have cross-checked that both train & test losses have the same dependence wrt the cropping size:
loss( 64x64 cropping) ~ (3-4) * loss (256x256 cropping)

They do not change (very little) if I modify (x2, /2) the batch size, the number of images in the datasets.