Hi,
I'll try to rephrase my question. In fact, the title is not in bold and the size of the font is not taken into account.
When you want to use a sans serif font you should also set one which isn’t the case in your setup.
\loadtypescriptfile[mathdesign]
\definetypeface [mainface] [rm] [serif] [palatino] [default]
\definetypeface [mainface] [tt] [mono] [modern] [default] [rscale=0.9]
\definetypeface [mainface] [mm] [math] [xits] [default]
\setupbodyfont[mainface,11pt]
Wolfgang