[metapost] Problem with defaultscale

[Oliver Buerschaper]

Dear MetaPost wizards,

I'm trying to change the default font size MetaPost uses for labels  
but the following code keeps giving me trouble:

---
beginfig(1);
     defaultscale := 6;
     label(btex Hallo $\sum$ etex, origin);
endfig;

end
---

Whatever the value of "defaultscale" the font size of the label  
doesn't seem to be affected :-( My typesetting command is

mptopdf labels.mp

What am I missing?

Oliver