Features that are "not complicated" can actually very easily be "very complicated" at scale. Which Instagram does have. 500 million users, every single day.
If you need several millions of lines of Python to do what Instagram server does, the code is bloated.
My bet is that they let too many Java devs loose on the code base, without experienced Python devs reviewing the commits and managing the deluge of unnecessary classes. I've seen it happen before.
>If you need several millions of lines of Python to do what Instagram server does
I have this feeling that you're probably not all that aware of 95% of what their code actually does, and thus probably not in a position to make judgements as to whether their code base is truly bloated relative to what it does.
