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Text sixe is controlled on the system level and you can adjust it in the settings: General / Font Size

What if I only want a smaller text in Tweetbot and not all over iOS?

You're out of luck.

I'm not. According to a FAQ published today, Tapbots is working on a solution … :)

But I really don’t want to have the same font size as the rest of the OS.

We hear you and are working on something, stay tuned.