Mathematicians do very large proof steps, and so can the computer these days. If you do something like (proof by auto: thm_1 thm_2 thm_3) and it goes through, then I know why the proof works (because of these theorems), and this is similar to how a mathematician understands something. Syntactic brevity is indeed an indicator that you understand the proof. The better you understand something, the more concise you can formulate and prove it.