Here is a simple test illustrating two bugs in \normalsize:

\text{a}, \text{\normalsize a}, \frac{\text{a}}{\text{\normalsize a}}

First, there's a space before a in \text{\normalsize a} (in the denominator). Second, the size isn't actually changed! Here's how it's supposed to render, in LaTeX: (note larger a in denominator)

I'm worried that I don't understand how sizing nodes are meant to behave in KaTeX. This comment worries me a little:

groupTypes.sizing = function(group, options) {
    // Handle sizing operators like \Huge. Real TeX doesn't actually allow
    // these functions inside of math expressions, so we do some special
    // handling.

How are sizing commands supposed to work in math mode in KaTeX, given that they don't work in LaTeX? In any case, the way they're working in text mode doesn't match LaTeX, so that's presumably a bug.