Maths symbols don't scale up
By default, the “large” maths symbols stay at the same size
regardless of the font size of the text of the document. There's good
reason for this: the
cmex fonts aren't really designed to
scale, so that TeX's maths placement algorithms don't perform as
well as they might when the fonts are scaled.
However, this behaviour confounds user expectations, and can lead to slightly odd-looking documents. If you want the fonts to scale, despite the warning above, use the exscale package — just loading it is enough.
Source: Maths symbols don't scale up