I wrote a longer post a few months ago.[1] The tl;dr is a) computer aided design and manufacturing b) aspherical elements c) fluorite glass d) retro focus wide angle designs and e) improved coatings. Mirrorless lenses also beat slr lenses because they are much closer to the film plane — of course rangefinders and other classic designs never had this problem to begin with. 1: https://news.ycombinator.com/item?id=42962652

Edit: this is just for prosumer style cameras. If you look at phone sized optics that’s a whole other ballgame.