From the Super Mario Wiki
An Expansion Block is a block that resembles a pink Egg Block in Super Mario World 2: Yoshi's Island, Yoshi's Island: Super Mario Advance 3 and Yoshi's Island DS. However, it does not release eggs. Instead, when struck from below, an Expansion Block expands to help a Yoshi reach greater heights. However, after a set amount of time, Expansion Blocks shrink back to their normal size.