No particular reason. I'm going to try different patch sizes. Regarding the batch size, right now I'm using 32. Discarding chunks without mass, I get a dice score of 0.8 approx. which isn't really great, but I still have many things to try. And I'm checking recall and precision (and specificity) at the end of each epoch.

I'm going to try a bigger patch size and change the data loader as you suggested. Thanks for your help!