The difference comes down to syntax vs. semantics. The problem with assembly isn't its syntax (mov eax, ebx is fairly readable), it's that the underlying semantics are too low level. Exploring new abstractions/semantics to use is very useful (local variables, first-class functions, algebraic data types, etc.), but optimizing solely for source program size leads to a language that's great for code golf and not much else.
Not sure. I mean yes you can abuse some languages and fit an entire program on 1 line (see any golf project). But I can generally grok a 100 line scala program in the same time as a 100 line Java program, and the scala program is usually about twice as dense. (So I am groking scala twice as fast).
I generally find this to be true for myself, too. Although I would add the qualifier that I find Java to be an exceptionally wordy language, for lack of a better term. It seems to take a lot of talk to do a little in Java even compared to other object oriented languages.
Outside of J/K/APL few languages truly attempt to optimize for source size. Few people truly, deliberately optimize around source size outside those language programmers, demoscene types, and mathematicians/engineers-turned-programmers.
Add Ruby to the list. Many gems (and frameworks) advertise with how much you can pack into a single line of code, at the expense of seemingly impenetrable magic happening behind the scenes.
If not, you clearly see some gain in more terse syntax.
Seems like a good thing to explore, although perhaps it should not be one's sole focus in life.