Well, I don't know what to tell you....
You, yourself, saw something change in how messages are rendered -- same AquaMail version -- after the device updated to Android 5.0.
From my perspective, that's exactly what happened. A change in how the system code works (there are other changes, too... it's a whole new OS version, after all).
On the other hand, I'm not seeing any difference on my Nexus 5 -- at least nothing that would really stick out.
I just tried a few plain text messages, and they all go all the way to the right edge when opened. When I change font size (volume keys), there is reflow, also going all the way to the right edge.
Does your message have anything beyond what's visible (if you scroll down) -- some long URL, anything that would give it the larger width?
Oh, do you happen to have "auto-fit" enabled (under Message view)?