If I recall correctly (I've played it years ago), efficient mining methods make use of 1x1 branches and optimized spacing between branches, which exposes more potential ore spots in less time than a simple 2x1 tunnel. Someone solved the optimization problem and calculated the optimal branch spacing to minimize the time spent on removing the blocks per one block of ore.

Feather mining (the style that uses the 1x1 branches) is more efficient per block mined, but it requires a lot more attention compared to a straight 2x1 hole because of all the turning you need to do. So 2x1 is what most people do. People who have enough resources to make beacons usually speed-mine instead, though. That's because with a beacon giving you Haste II and an Efficiency V pickaxe, you can mine stone blocks instantly, allowing you to clear out 2-tall spaces rapidly. This destroys more blocks per ore but is much faster than anything else in terms of time. That said, in the 1.18 update, diamond ore can only be found at levels where deepslate generates instead of stone. Deepslate cannot be instant-mined, so caving is once again the most efficient way to find ores. I consider this more fun than the old style of mining, so I'm glad they made this change.

Straight mining also makes you pass through more chunks (16x16 section of the world) per block mined; since ore-gen happens based on chunking this gives you better odds.