I imagine it’s supposed to be for accessibility.
iOS 15 supports per-app dynamic text sizing, but I don’t know if their app does. I think that’s the way forward for those that need it, not making everything massive by default!
TSB tried it a while back and everybody complained then in the reviews that it made checking their balance difficult as anybody in a 100 metre radius could see what it was! So they gradually rolled it back too.
I expect RBS will do the same.
Yes, I was one of those who complained about that. The font was ridiculously large, and it was impossible to use their app in a public place as privacy went out of the window.
RBS kicking out the updates again
New QR code payment or link based payment option (using payit)
Change your address in all 
finally
And a carbon footprint thin
Had a quick play with this earlier! Worked pretty well!
Another change not included in the release notes is a tweak to the main screen text sizing.
It’s now back to something much more like the version before they made everything large, and is at a “sensible” default size again.
I haven’t tested if they are supporting dynamic text size for accessibility on iOS but it looks as though they may be, from the layout of the page.
That’s the most useful change of this update in my opinion, and if you compare the NatWest app (which hasn’t yet been updated) with the RBS one the difference is obvious.
Still no sign of dark mode though!