Solved by tom in post #2 
              
              
                Well, here, you have a batch size of 4 and one of 2 and set up MSELoss to average over the batch items. 
If you concatenate, you take MSELoss to average over all 6 items and it effectively computes the weighted average (4*8630+2*2850)/6 = 6703. 
When using a typical setup with PyTorch’s DataLoader, …