Thanks Thomas for your response.
Up to my knowledge, the gradient of loss function w.r.t an input pattern in .eval() mode is independent of  other input patterns in the mini-batch. In such a way as you mentioned the number of summation would increase by larger batch size but these summations can be categorized into independent calculations each corresponding to an individual input pattern. Thus I expect the calculations result in the same gradient. Do you have any idea about it?