<?xml version="1.0" encoding="UTF-8"?>

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <mi mathsize="small">A</mi> 
  <mo>+</mo> 
  <mi mathsize="big">A</mi> 
  <mo>+</mo> 
  <mi mathsize="3ex">A</mi> 
  <mo>+</mo> 
  <mi>B</mi> </math>