if there was a uniform land distribution,why should the seasonal contrast be larger in the southern hemisphere

Answer 1

Because the seasonal contrast is not just down to land distribution. As you are aware, the obit of the Earth around the Sun does not describe a circle but an ellipse. During the southern summer the Earth is slightly closer to the Sun than during the northern summer, when the planet is on the other side of the ellipse.

Hence the Earth is receiving more solar radiation during the southern summer and less during the southern winter.

Answer 2

Because the amount of land (continents) and the amount of water (oceans) has an effect on the seasons.

Because the Earth's orbit around the sun isn't uniform, the Southern Hemisphere is indeed, closer to the sun during its summer: 91.5 million miles vs. 94.5 million miles, which is about 3 million miles closer. As a result, the Southern Hemisphere, which includes most of South America, receives about 5% more energy from the sun during its summer. However, it isn't true that they have a warmer summer than the Northern Hemisphere, which includes the USA.

The Southern Hemisphere contains much less land mass and much more ocean than the Northern Hemisphere. Water takes longer to warm and cool than land since it has a higher heat capacity. Therefore, temperatures are unable to warm up as quickly during the Southern Hemisphere's summer as they are in the Northern Hemisphere.

Because the Southern Hemisphere is dominated by oceans, maritime air masses -- moist air originating over water -- control the region's weather, keeping temperatures cooler during the summer and milder during the winter. In contrast, continental air masses -- dry air originating over land -- control the Northern Hemisphere's weather, allowing temperatures to heat up considerably during summer and get very cold during the winter.