Why one uses 了 and the other one doesn’t? 我每天吃吃睡睡，从来不运动，吃饱就睡，醒了，就吃，怎么还是不胖。


I wonder why the writer didn’t add 了 after “吃饱” whereas they added 了 after 醒.

I know what 了 indicates here, it’s just that I don’t know why the writer didn’t treat “吃饱就睡” and “醒了就吃” equally.


  1) Is it for the sake of “even amount of characters (4 - 4)”?


Then...


  2) Is it okay if I add 了 after 吃饱 > “吃饱(了)就睡”?



One trivial question to add :



  A native once corrected me for not putting 都 after 每天, then why
  doesn’t this one have 都 in it though?
Overall, your analysis is correct. It's indeed for the sake of “even amount of characters (4 - 4)” in this case. It might be more obvious if it writes: "吃饱就睡，醒了就吃" or "吃饱就睡，睡醒就吃".

吃饱了就睡 is also correct.