From the Super Mario Wiki
Big Whomps are a larger sized sub-species Whomps. They are rather rare types of Whomps, having only two appearances. They are similar to Super Thwomps, as they can also break through Brick Blocks.
New Super Mario Bros.
One Big Whomp makes an appearance in New Super Mario Bros. for the Nintendo DS, where it resides in the World 3 castle. It can be defeated through use of a Ground Pound. It behaves in the same way as normal Whomps, only it can walk around freely instead of staying stationary in the background and falling on Mario or Luigi.
New Super Mario Bros. 2
A Big Whomp in New Super Mario Bros. 2
Big Whomps reappear in New Super Mario Bros. 2 for the Nintendo 3DS. They appear in World Mushroom-Castle, like the other games it will need a ground pound to defeat, though a few are seen in a World Star level. In this game, Big Whomps are again able to walk around, but they can also be seen in the background. Once a Big Whomp in the background falls, there can be Coins, Roulette Blocks or even a Red Ring behind them.
- ^ New Super Mario Bros. 2 Prima eGuide, "Common Critters" tab.