Inconsistent Large text in iOS 11.4

Hi everyone,
I have just upgraded my iPhone SE to iOS 11.4 and it seems to be running ok so far, but I am noticing inconsistencies with the large text option under Accessibility settings. Parts of the interface (like the Settings, Email app...) seem larger to me on the highest setting than iOS 10, but the actual text of email messages and other apps (like the RSS Reader Lire or in Whatsapp messages) seem smaller.
Is there anything I could do to fix this?
Thanks,
Claire