Is that common?

Nowadays I just don't know anynore. I find toilets where it's swapped.

I also want to say that I grew up with toilets where the bigger button is the one for your more common usecase - liquid waste. Oftern the smaller button is set up to push the bigger button as well - to clean solid waste.

If using a new toilet, I always try out the bigger button first, since it's easier to press and typically uses less water.