Increase size of sum (uppercase Sigma), prod, etc

The existing sum visual test case looks closer to KaTeX and MathJax,
see GitHub commit comment (copied below).

Before:
<img width="60" alt="before" src="https://cloud.githubusercontent.com/assets/225809/14303182/4aafeb52-fb5d-11e5-9698-872fdc6b0f87.png">

After:
<img width="64" alt="after" src="https://cloud.githubusercontent.com/assets/225809/14303181/4aaedd16-fb5d-11e5-91cc-a6f3e7395440.png">

KaTeX:
<img width="83" alt="katex" src="https://cloud.githubusercontent.com/assets/225809/14303180/4aadea32-fb5d-11e5-8921-e1f2a920d02c.png">

[MathJax](http://mathb.in/55041):
<img width="71" alt="mathjax" src="https://cloud.githubusercontent.com/assets/225809/14303179/4aadb846-fb5d-11e5-8041-e7e50a6f32d4.png">

src/css/math.less

diff --git a/src/css/math.less b/src/css/math.less
index fcaa23d..5fa0acc 100644
--- a/src/css/math.less
+++ b/src/css/math.less
@@ -87,7 +87,7 @@
   }
 
   big {
-    font-size: 125%;
+    font-size: 200%;
   }
 
   .mq-roman {
@@ -291,6 +291,8 @@
   }
 
   .mq-large-operator {
+    vertical-align: -.2em;
+    padding: .2em;
     text-align: center;
 
     .mq-from, big, .mq-to  {