I know this sounds crazy but it kind of makes sense to me. It's a large boulder in the context of things you might find obstructing the road, but it's a small boulder in the context of all boulders. Still pretty hilarious to read.
Yeah after seeing it so many times a "large boulder the size of a small boulder" is just the size of the boulder in the picture of that tweet to me :)
But I think it was a typo, and they meant to say "A large boulder the size of a small car"
Yours was a long comment the size of a short comment :-)