Along with the more recent "flat" UI look came an emphasis on making everything on the screen look smaller. Tinyer text, etc. Which I personally dislike. I prefer big, chunky stuff on the screen like it used to be. Much easier to see. The newer stuff is probably a big reason why so many journalists and editors for major news outlets allow so many typos to slip through. It's harder to read stuff off of more recent mac screens! Hopefully the dark mode we'll soon be getting will help. I've been wanting dark mode for a long time, and fortunately there is an app called mouseposé that has a "flashlight" feature that has toned down all the harsh whites on the screen for me for years. I would have been a really miserable mac user for years if it weren't for mouseposé.
Cocoa UI elements are still the same exact size as the first release of Mac OS X. Why do you think they are smaller?