Isn't it common for Japanese apartments to be super tiny though? As in enough space for a mattress and not much more.

Yes, but if you want to have adequate supply in a very space limited area, that's going to be one of the natural compromises.

I'm no expert, just a virtual tourist via YouTube.

According to (1) average Tokyo apartment size is 20 square meters or a little over 400 square feet vs 700 square feet in San Francisco as found from a quick Google search.

Considering some Japanese rent and live in Cyber cafe cubes that are even smaller I'm not sure how fair a comparison really is between the two. Japan in particular seems to have small appliances geared towards smaller footprint apartments.

https://www.all-about-teaching-english-in-japan.com/Tokyoapa...

Japanese people are also smaller than Americans!

Joking aside, the lifestyle is also different. Tokyo and HK where I live are known for tiny apartments, but they're also cities where residents spend far less time at home, offering cheap options for eating out, many places to hang out outside home, reliable transit, and extreme safety from crime at all times of the day. I'm far more comfortable in a tiny HK flat than I would be in the same size in SF. Thankfully I have the luxury of a decent sized space in both now.

Sadly you are correct about Japanese vs American sizing. But beyond that hanging clothes to dry vs having a dryer, inbuilt grills on stoves, rice cookers, dishwashers being rarer, etc all add up to more efficient use of space.

20 sq m is roughly 215 sq ft, not 400 sq ft as mentioned in the post.

215 vs 700 is a very different story from 400 vs 700.