The mstyle element is used to make style changes that affect the rendering of its contents. MathML Fundamentals
<math mathsize="45"> <mrow> <mo>(</mo> <mfrac> <mi>a</mi> <mi>b</mi> </mfrac> <mo>)</mo> </mrow> <mo>+</mo> <mstyle maxsize="1"> <mrow> <mo>(</mo> <mfrac> <mi>a</mi> <mi>b</mi> </mfrac> <mo>)</mo> </mrow> </mstyle> </math>