A margin call occurs when margin level already touching margin call level threshold, there are broker provide margin call level at 100% and stop out the same, but there are the broker that provides margin call level 100% and stop out level 50%, when you trading with bigger lot size it will make your margin level being narrowed and close to margin call level, if the price against the order, it will reducing margin level and when touching margin call level, the broker will close order automatically.