Why smaller batch size results better results? With the epochs increased, the G loss went up?

There are 8k+ 256*256 images in my datasets, I set batchsize=32 and trained with 4 12GB GPUs, but it's not as good as I set batchsize = 4 and trained with one GPU,neither the training speed nor the image quality. Why a smaller batch size results better results? Is there a best batchsize?
Beside,I set batchsize=4, when epoch>70, g-loss went up obviously,and qualities of generated images went worse.Why did this happen? (Training process was interrupted when epoch = 54, I reloaded weight files and optimer states from epoch 53 )
Thanks!