It's tricky to say definitively which is "bigger" because both plateaus and coastal plains can vary greatly in size. Here's why:

* Plateaus: These are flat, elevated areas of land. They can be massive, like the Tibetan Plateau (the largest in the world), or relatively small.

* Coastal Plains: These are low-lying areas of land that extend along coastlines. They can be vast, like the Atlantic Coastal Plain in the US, or quite small, like the narrow strip of land along some Mediterranean coasts.

So, it really depends on the specific plateau and coastal plain you're comparing.

Here's a helpful way to think about it:

* Area: Plateaus generally have a larger overall area, but it depends on the specific examples.

* Elevation: Plateaus are always elevated, while coastal plains are low-lying.

Ultimately, the best way to determine which is bigger is to compare the specific geographic features you're interested in.