Android 4.4 has substantial changes to the system component for displaying HTML.
Pretty much every single app that used this component (including K9) was affected, and had to find workarounds (yes, Google doesn't care).
As a result, Aqua lost the "+/-" message zooming buttons. However, double tap should still work -- and is handled by the system.
Also note that there is a new zooming gesture in 4.4: "one-and-a-half" tap and drag...
It's kind of hard to explain, but I'll try: tap - release - tap - drag your finger up or down. This is system-wide, and also works in Chrome.
