During their childhood, I assume they had the same height. Maybe Saeran walked a little stooped, but then, as adults Saeyong became a few cm tall. Thinking about how their daily lives went before they separated (Saeran with a weaker condition than his brother, staying more at home) and after (Seven going to training at the agency, thus eating better and exercising more), I assume this height difference occurred.

Related to this, his complexity is also different: Saeyoung being more muscular (considering his constant outside missions) and Saeran more slim (he spends the whole day programming, barely eating and sleeping). For the same reasons, even their skin color is a few shades different. Saeyoung is tanner than Saeran, though I think after Saeran leaves that place he could also get more tanned, thus closer to Saeyoung's skin color. You can see more in my next > post < where I talk about him :D

Now, their smile. Saeyoung has an asymmetrical smile and tends to show his canines, primarily because I like it, but I think he is more used to smile and a more playful smile suits him better (I can imagine him using it to conquer the hearts of his rivals haha). On the other hand, Saeran is not used to smile nor feel confident doing it, so I give Saeran an innocent smile, more timid. With time, this can change obviously.

Extra: Eye color.

Due to the elixir, Saeran's eyes change their color to one similar to that of the elixir. I theorize this tone can be banished with time as long as he doesn't take that poisson anymore. So it goes like this: he has the same yellow eyes as his brother before Mint Eye, then they turn mint-colored because of the elixir, and after he leaves Mint Eye they turn hazel, similar to their original color but retaining green tones.