Have a look at this link:
http://www.codeproject.com/Messages/1908296/Font-size-increases-on-Postback.aspx[
^]
It says these could be the problem.
1.You have a literal control then you set his value to this Literal1.Text =”<H1>My H1” when page load was trigger.
2.You have a label and you set his value to this Labe1.Text = “This is a <B>Bold”.
3.GridView or Datagrid Item has a HTML value with no closing Tags.
4.You have generated a DHTML code with no closing Tags. Eg. InnertHTML or OuterHTML.
Check if it helps.