[NTG-context] Why do size changes affect my math font?

[Brooks Moses]

Consider the following simple document, with is a minimalist version of the 
slides that I'm currently working on:

   \starttext
   The variables are $u$ and $p$.
   \stoptext

This works fine; the text is in roman and the math in italics, just as I 
desire.  However, I need a larger font for my slides, and thus I'm also 
using a font size command, like so:

   \starttext
   \tfc
   The variables are $u$ and $p$.
   \stoptext

This causes the math to be in roman instead of italic.  (It's not in "text 
font" per se; an \it after the \tfc will italicise the text but not the 
math, creating a quite odd effect.)

Is there a reason for this?  What's the cleanest way to fix it for all the 
various size commands?

Thanks,
- Brooks