First one is a steep straight line followed by a gentler straight line. This is because the same volume of water will fill up the height of a narrow container more quickly than a large one. Within the narrower section, there is no change in diameter, hence straight line.

Second one is curve like that for the same reason. Bottom part is narrower, middle is the fattest (so the graph slows down to lower gradient) and top part is back to narrow so larger gradient again. Must be a curve.